The equipped creature's power is checked only during the declare blockers step. If its power becomes greater than 3 after blockers are declared, it still can't be blocked. Similarly, if its power becomes 3 or less after it's been blocked, this won't make it unblocked.
When this Equipment enters, create a Treasure token. (It's an artifact with ", Sacrifice this token: Add one mana of any color.")
Equipped creature can't be blocked as long as its power is 3 or less.
Equip (: Attach to target creature you control. Equip only as a sorcery.)
, Sacrifice this token: Add one mana of any color.")
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