Lethargy Trap

Instant — Trap

If three or more creatures are attacking, you may pay rather than pay this spell's mana cost.
Attacking creatures get -3/-0 until end of turn.

2009-10-01:
You may ignore a Trap’s alternative cost condition and simply cast it for its normal mana cost. This is true even if its alternative cost condition has been met.
2009-10-01:
Casting a Trap by paying its alternative cost doesn’t change its mana cost or converted mana cost. The only difference is the cost you actually pay.
2009-10-01:
Effects that increase or reduce the cost to cast a Trap will apply to whichever cost you chose to pay.