Steel Overseer

{T}: Put a +1/+1 counter on each artifact creature you control.

Etched Champion

Metalcraft — Etched Champion has protection from each color as long as you control three or more artifacts.

Dispatch

Tap target creature.
Metalcraft — If you control three or more artifacts, exile that creature.

Plains

Memnite

Golem's Heart

Whenever a player casts an artifact spell, you may gain 1 life.

Glint Hawk

Flying
When Glint Hawk enters, sacrifice it unless you return an artifact you control to its owner's hand.

Perilous Myr

When Perilous Myr dies, it deals 2 damage to any target.

Myrsmith

Whenever you cast an artifact spell, you may pay {1}. If you do, create a 1/1 colorless Myr artifact creature token.

Tempered Steel

Artifact creatures you control get +2/+2.

Ancient Den

Mox Opal

Metalcraft — {T}: Add one mana of any color. Activate only if you control three or more artifacts.

Signal Pest

Battle cry (Whenever this creature attacks, each other attacking creature gets +1/+0 until end of turn.)
Signal Pest can't be blocked except by creatures with flying or reach.