The volume of swimming, biking and running that triathlon requires leaves little time for the gym, so triathletes should approach strength training conservatively. This approach will allow you to continue to train hard without feeling too beat up from the weight training.
These exercises focus on your posterior chain (back, glutes, and hamstring) and will help counteract the typical muscle imbalances commonly found in triathletes. Forty-five minutes in the gym twice a week is all that you need. If you have some big races coming up, continue to strength train, just reduce the intensity of the workouts.