There are two triathlon races and both are unlocked upon completion of Yay-Ka-Boom-Boom when Las Venturas becomes accessible. Before that you'll receive a "The Cluckin' Cock isn't ready yet" message at the starting points. The races are available ONLY on Saturdays and Sundays. The name of the contests is something that you'll find out if you look at a certain competitor. Be sure to have some fat so that you don't burn Muscle. The order of the events are swimming, biking, and running. If you complete the burglary missions you'll get the ability to sprint infinitely making these contests a lot easier to win.
The starting point of the shorter race is at the lighthouse in Santa Maria Beach in Los Santos. This race goes through Angel Pine and ends South of San Fierro at the radio tower on top of Missionary Hill.
The starting point of the longer race is at Fisher's Lagoon in Red County just West of the small town of Palomino Creek. Its route goes West all the way to San Fierro and then back East to Las Venturas through the desert. The finish line is in central Las Venturas.
The detailed guide below is for the shorter race.
Once the race starts run and start rapidly pressing . You will swim around the lighthouse and near a shore. Don't be discouraged if during the middle of this leg you get passed and fall to last, as you can make up a lot of ground near the shore. Once you are out of the water get on the mountain bike.
After you get on the bike, as before, rapidly press . The path will go from the paved two lane road to a dirt trail. Enjoy biking along and through the countryside and eventually through (literally!) the town of Angel Pine. Be in 1st or 2nd place by the time you cross the bridge near Missionary Hill. Once you get to the 1st intersection near the golf club, get off and start the next leg.
After you get off the bike, run Forrest, run! You'll begin running up the hill. If you begin to slow down due to fatigue then stop, look behind you, and if there aren't any others around take a 2-5 second rest. The Race ends when you get to the Radio signal next to the TV Building.
For winning the shorter race you get $10,000 and if you needed to burn some excess fat it's most likely gone.
The longer race comes with a $20,000 prize.
Completion of the races is not required for 100% completion.