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

Your opponent still chooses which attacking creature each creature they control blocks.

Predatory Rampage doesn't force any creature to attack.

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.

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, a planeswalker controlled by that player, or a battle that player protects, 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.