I don't see anyting wrong with it either. It might not be wise to keep it up for longer than say 2 months (give or take)- two days inbetween might not be enough rest for your muscles if you're doing a really heavy day on Monday. remember- you build muscle after the workout, not during.
I've started doing one muscle at a time, twice a day. In the morning, I'll do a major muscle group like back, then late in the afternoon I'll do biceps. Or Chest early in the day, then abs in the evening. I've been keeping this up for two weeks now, and will continue for another two before I change it up. I like it better than "one day on, one day off".
I have a question, though- you say you've been doing three sets per muscle group? Is that it? Or do you mean you do a few different excercises, with 3 sets each exercise, and about 8 reps per set?
I like to do at least 8-10 total sets per muscle. It varies, depending on how heavy I'm lifting & how many reps I'm doing. I've recently started going with lower weight/higher reps (9-11 reps) and 11-12 sets. I was doing high weight, 4-6 reps max, with about 3 sets per exercise.
And every few months, I'll take a week off, too.