Only creatures you control when Predatory Rampage resolves will get +3/+3. Creatures that enter the battlefield or that you gain control of later in the turn are unaffected.

Each creature your opponents control blocks if able, even if that creature wasn’t on the battlefield or wasn’t controlled by an opponent when Predatory Rampage resolved.

Predatory Rampage doesn’t force any creature to attack.

Each creature controlled by an opponent blocks only if it’s able to do so as the declare blockers step begins. If, at that time, the creature is tapped, is affected by a spell or ability that says it can’t block, or no creatures are attacking that player or a planeswalker controlled by that player, then it doesn’t block. If there’s a cost associated with having the creature block, the player isn’t forced to pay that cost, so it doesn’t block in that case either.

Your opponent still chooses which attacking creature each creature he or she controls blocks.

If there are multiple combat phases in a turn, each affected creature must block only in the first one in which it’s able to.