(Now that Walking Ballista has been purged from pioneer this deck is essentially dead, I'm leaving it available to access but just know it is a defunct deck.)

As a lover of Artifacts I've found Hardened Scales Affinity decks to be an incredibly fun experience, it is one of the few archetypes that doesn't rely on spamming Planeswalkers or two card instakill combos to win. Even though the Companion rule changes have reduced the effectiveness of Lurrus, this deck can still utilise Lurrus very well and because of this I've decided to keep the Nightmare Cat as a part of this deck.

Lets start off with the Scales that make this deck so effective. 

Hardened Scales and Winding Constrictor provide solid and consistent value in the form of an additional counter on all of our affected creature spells. Metallic Mimic is the third Scale and the most explosive of the three, it is entirely possible to play most if not all X spells in hand without paying any mana in a single turn thanks to Mimic. The tribal condition of Mimic does means that either the Constructs or Snakes of the deck enter without an extra counter, considering the power of the Constructs in 3 Scales they'll usually end up eating all of Mimic's extra counters. 

Now onto the X Creatures of the deck. 

Walking Ballista is one of the best cards in Pioneer and this decks best win condition, it is a hard threat to deal with thanks to its ability to deal damage and add counters to itself at instant speed. Hangarback Walker is the second best X Creature in 3 Scales, its ability to essentially negate non-exile removal and the fact it is able to continue growing itself over time makes it hard to safely remove from the battlefield. Stonecoil Serpent is the weakest of the three threats but it still remains capable of disrupting both aggro and control decks thanks to its protection from multicoloured and reach ability. 

One of Hardened Scales' biggest weaknesses is its lack of generating long term card advantage. Lurrus while weakened is still provides a great advantage in longer games, graveyard recursion even just once with this deck can turn the game in your favour. Sign in Blood is the now the second best cheap Black draw spell in Pioneer thanks to printing of Village Rites, for the cost of one black and sacrificing a creature to draw to cards at instant speed Village Rites is in my opinion the definitive draw spell for this deck. Evolutionary Leap is an inconsistent source of draw, but the fact that it guarantees a Creature and can fizzle a kill spell or bounce spell makes it a worth while inclusion. There is a question is of what ratio of Evo Leaps to Village Rites should be included, currently I'm running two of each but if village rites proves to be stronger I will probably remove one or both of the Evolutionary Leaps. Bonders' Enclave is not the most efficient draw and its condition may be challenging to fulfill in the face of consistent removal, but lacking anything else it can provide some much needed card draw. I have replaced Bonders' Enclave with Castle Locthwain because it is more consistent and the life loss is not so much of a problem thanks to the replacement of Sign in Blood with Village Rites. 

Thoughtseize is the best hand disruption in Pioneer, if you're playing deck with Black and you can afford the exorbitant price, run this card. Fatal Push is fantastic against aggressive decks, and because we're running 4 copies of Fabled Passage and have cheap ways to sacrifice creatures it's very likely that ends up hitting higher cost and higher impact Creatures. 

These two cards are terrible top decks and fantastic value engines, I think one of each is a pretty acceptable inclusion in this deck. The Ozolith can turn X Creatures being destroyed into sustained value, it can be a win condition but is far more susceptible to hard and soft removal hence the singular inclusion. Animation module is another card that can either be dead or a source of continuing value, it can give an X Creature an extra counter per turn and generate a wide board but its mana investment is often detrimental to 3 Scales' aggressive gameplan. Personally I think that the Ozolith is an overrated card, maybe I'm wrong but in Pioneer and even Modern I don't think you want to include any more than two in a deck, it's a weak card by itself and does terribly against most decks. Including either may give you a slight advantage in fringe situations, for now I'll be running a single copy of The Ozolith and forgoing Animation Module.

The land base is pretty standard for its colours and archetype.

Onto the Sideboard,

Scavenging Ooze is great against Graveyard and Burn decks, it does everything you need, it eats the Graveyard, gains life, and uses counters.

Damping Sphere efficiently deals with Lotus combos, Paradoxical Outcome combos, and Izzet Phoenix. Sorcerous Spyglass gives you the advantage in a mirror match and shuts down pesky Planeswalkers, it may be a good idea to run two of these considering the prevalence of Planeswalker decks in pioneer. I have replaced Tormod's Crypt with Soul-Guide Lantern, the Lantern's extra utility has proved that it is a more consistent form of graveyard removal.

Savage Summoning lets you troll irritating flash and control decks, it makes your next creature instant speed, uncounterable, and gives it an extra counter. However, Savage Summoning is on notice, it hasn't performed as well as I wanted it too and there might be better sideboard options against control matchups. Assassin's Trophy is one of the best hard removal cards in Pioneer, it is extra removal for almost every threat. There is a debate to be had as to whether Assassin's Trophy or Abrupt Decay is the better form of removal. My suggestion is this, in a vacuum Assassin's Trophy is the better kill spell as it hits literally every possible target save cards with Hexproof and Protection, if you know you're going into a control heavy environment then Abrupt Decay is usually the better option. Necromentia is the new best tool for getting rid of troublesome combo pieces and spells before you need to worry about them, and the downside of potentially giving your opponent a few 2/2 zombies doesn't significantly affect 3 Scales. 

So that's the list as so far, if I've missed anything or made some grievous errors in the creation of this deck let me know. I'd also like to talk about a few cards that I haven't included in this list, and explain why they may be good substitutes or necessary additions in particular meta environments.

Extinction Event is the best Black board clear in Pioneer change my mind!!!! jk aside it is a pretty effective clear against a lot of Pioneer creature decks, its only downsides are its mana cost and that it will exile your permanents as well. Hour of Glory disrupts gods, which may be relevant in today's metagame considering Heliod has been a fairly effective force as of late. Vraska's Contempt hits Creatures and Planeswalkers and gains life, all of those effects are good against most deck types. Pharika's Libation is questionable but it forces the opponent to Sacrifice either a Creature or Enchantment at instant speed, a particularly great effect when facing Lotus Breach, Heliod Combo, and Creature decks that don't go wide. Legion's End is a potentially invaluable addition to this deck as it can severely disrupt Creature Aggro decks and can give one the advantage in a mirror match. Trial of Ambition is the weakest of these options, as there is no way to get it into the graveyard after it has been cast to get that Lurrus value. 

Pick the Brain, Dispossess, and Lost Legacy are Necromentia with worse conditions, Pick the Brain requires Delirium and that the opponent is holding the card you need them to and Dispossess only hits Artifacts, Lost Legacy fails to deal with Lands or artifacts, these are massive downsides and the reason why I've chosen to include Necromentia in the deck list in lieu of the other options. Brain Maggot it susceptible to Removal but synergises well with Lurrus and doesn't cost potentially valuable life unlike Thoughtseize, it does however cost more mana and often dies horribly. 

Barrier Breach is especially effective against Enchatnment centric decks and avoids single target protection and instant speed single target hexproof, if you know that you're going in to an Enchanment heavy meta environment you will probably want this card in your Sideboard. Mystic Repeal is particularly great against gods, and the fact that it gets rid of an Enchantment Permanent at instant speed and without putting it into the Graveyard might be relevant in the post-Ikoria release meta game. Natural State is solid Enchantment and Artifact removal, but I think it is out valued by most other cards with similar effects. 

The lack of life gain in this deck may be an issue when going against aggressive decks, Shadowspear makes big X Creatures a great source of life gain and as a bonus gives them trample. It also gives the deck an opportunity to target Hexproof and Indestructible permanents, an effect that may see it weasel its way into this deck at some point.