If you just look in the UFC 3 Menu you’ll see that there is an option to purchase Bruce Lee as a playable fighter in the game. While the fighter can be purchased you can also unlock him for free by simply completing the Career Mode in UFC 3 by completing the G.O.A.T. challenges.
It is simply not enough to complete the game and earn the championship in your weight class to unlock Bruce Lee. You must win the championship in your division, win a second belt in another division, and then complete the G.O.A.T. challenges.
Many of the challenges you can complete simply by playing the game. Others will boil down to your success in the Octagon. Easy G.O.A.T. challenges to complete are the earnings, fights, and successful title defense challenges.
Once you’ve completed the career mode and you’ve been acknowledged as the G.O.A.T. you will earn Bruce Lee as a playable character and he can be chosen when playing through the Lightweight, Welterweight, and Featherweight classes.
It’s worth noting that you can complete the Career Mode on any difficulty, and change the difficulty mid-Career and still earn Bruce Lee. If you’re having trouble with different opponents on your way through the ranks and need to bump it down a notch in difficulty just to earn Lee, you can do so.