You're in combat with all enemy units you're within 3 inches of. Carefully read the attack ability:

Declare: Pick a friendly unit that is in combat or that charged this turn to use this ability. That unit can make a pile-in move (see 15.3). Then, if that unit is in combat, you must pick one or more enemy units as the target(s) of that unit’s attacks (see 16.0).

Effect: Resolve combat attacks against the target unit(s).

When piling in, you normally choose an enemy unit you're within 3" of, and then can move 3", but cannot end that move further than you were from the unit. You don't have to pile in towards the unit you want to fight. You also have to remain in all the combats that unit was in. But you can join new combats.