Which ability from the "Magic: The Gathering" Scars of Mirrodin expansion involves having at least three artifacts in play?

The ability from the Magic: The Gathering Scars of Mirrodin expansion that involves having at least three artifacts in play is Metalcraft. Metalcraft is a mechanic that rewards players for controlling a certain number of artifacts. When a player has three or more artifacts in play, Metalcraft abilities become active, often granting additional effects or boosting the power of certain spells or creatures. This ability encourages artifact-focused strategies and adds an extra layer of complexity to gameplay, creating interesting tactical decisions for players to make.