To unlock the UFC 13: The Ultimate Force in EA SPORTS UFC 3, you will need to progress through the game's career mode until you earn a legendary contract.
You start the game with a rookie contract and must progress through the game completing different challenges in order to make your way up the UFC ladder.
The levels are (there are trophies relating to each level of contract):
- Rookie
- Prospect
- Contender
- Superstar
- Legend
This trophy will unlock through natural progression in the career, although you will need to be able to win fights in order to progress, so set it at a difficulty that you are comfortable with.