QCan a follower with both Rush and Assail attack a reserved enemy follower on the same turn it comes onto the field?

AYes, it can.

QMy opponent has 2 copies of Phoenix Roost on the field, and their abilities are played in order, leading to an Urd and this follower coming onto my field. I use Urd's Fanfare ability to send an enemy follower to the EX area, thereby reducing the number of cards on my opponent's field to 2, before playing this follower's Fanfare ability. Is the condition "If there are at least 3 cards on their field" still satisfied?

ANo. By the time this follower's Fanfare ability is played, there are no longer "at least 3 cards on their field."

QCan you count cards in your opponent's EX area for this follower's Fanfare ability?

ANo, you cannot.