Whenever a creature enters, if there are two or more other creatures on the battlefield, exile that creature. Return that card to the battlefield under its owner's control when Portcullis leaves the battlefield.
The creature does enter, so any other abilities that trigger on it entering trigger. This follows the normal rules for timing triggered abilities: first, the current player puts their abilities on the stack in any order, then each other player in turn order does the same. Portcullis' own ability is included in this, so it could result in the creature being exiled after some abilities have resolved, but before others.