Whoa, I can't believe it, you have just received cataract surgery and you don't even wanna give your body enough time to recover and rest. As a matter of fact, catract is a serious kind of eye disorder, it could even lead to blindness you know? Now that you just had 3 weeks' rest and perhaps your eyes are not fully recovered. So, I strongly recommend that you stay away from tough sports for at least 2 months before you feel much better. Better yet, you should seek professional advice from the doctor and reexamine your condition!
Generally speaking, you should avoid swimming and severe exercise within a mouth after a surgery. And weight lifting is exactly a kind of "severe" movement, because weightlifting will suddenly increased intraocular pressure, thus format adverse effect to the eyes, then easy to cause some complications. And after a strenuous exercise, the sweat may flow into the eyes, this may against recovery. It is only after three weeks and I think you should avoid the sport. Even after a month, you also need to go to the hospital do some check, and make sure there is no problem before doing favourite things.
Usually, you will get recovery after a week of cataract surgery. However, you should have the good rest for the eyes. If you want to do the exercise, you'd better wait after a month of the surgery. Because there must be the sweat flowing into your eyes when doing exercise, you should prevent the eyes from infection again.