The attacking player chooses whether to tap a creature for an enlist ability immediately after they tap the creatures that they have chosen to attack with. You can't choose to enlist a creature later.

You may tap only one creature for an enlist ability of an attacking creature, and a single creature can't be tapped for more than one enlist ability.

When a player taps a creature for an attacking creature's enlist ability, that attacking creature gets +X/+0 until end of turn, where X is the tapped creature's power. This is a triggered ability that goes on the stack immediately after attackers have been declared in the declare attackers step.

Once a creature with power 3 or greater has blocked this creature, changing the power of the blocking creature won't cause this creature to become unblocked.

To enlist a creature, that creature must be untapped, it must not be attacking (even if it has vigilance), and it must have haste or have been under that attacking player's control since the beginning of their current turn.