The ability checks the power of each creature as it enters. It will take into account counters the permanent enters with and static abilities that affect its power (such as Flinthoof Boar's ability). After the creature is on the battlefield, raising its power with a spell (such as Titanic Growth), an activated ability, or a triggered ability won't cause this ability to retroactively trigger.

The creature's power is checked only to see if the ability triggers. It doesn't matter what the creature's power is when the ability resolves.

If Garruk's Packleader and another creature with power 3 or greater enter under your control at the same time, the ability will trigger.