My physical therapist wants me to box six 3-minute rounds before I do weight training. After boxing I feel pretty tired and of course not 100% energized, and I can't perform 100% in weight training due to this (squats, leg presses, etc). Is it good to box to warm up or should I just do light warm-up exercises and jump straight into weight training?
6 x 3 minute rounds seems a lot for a warm up. I would imagine you would be tired and unable to do a complete weight training program. What are your goals? and why did your personal trainer suggest this program? If your focus is on improving cardio, this might be right, but if it's strength training....you would normally do a light warm up, focus on the strength training and then some cardio.
I'd say it's pretty risky to ask someone to box 6 3-minute rounds before doing weight training. As you said, you feel pretty tired and don't feel like you have 100% energy to do the weights, and as a result you won't be getting the benefits out of it that you could if you were fresh. Not to mention if you're fatigued you're more likely to be sloppy and make a mistake, injuring yourself. I wouldn't do anything more than a round of shadowboxing for warmup, or perhaps a few minutes of rope skipping. Since boxing would be more of an endurance exercise, and lifting heavy weights will sap some of your endurance, doing the 6 3-minute rounds of boxing after the weights makes the most sense.