I have to agree with Atlantia .... Princess Bride.
But this is a close second for me. The 3-way sword duel in Pirate's of the Caribbean - Dead Man's Chest. It's in 2 parts. Doesn't really get going until the second half of part 1:
http://www.youtube.com/watch?v=FUcMtdYRsHg
http://www.youtube.com/watch?v=VgxoXXd3MaE&NR=1
