If you go by the exrx site, putting in 212 as your 5 RM gives you an estimated 1 RM of 239, and according to the table below, your 3 RM should be appx 90%-93% of your 1 RM, so that's 215 - 222 lbs. According to the HST calculator, if you have a 5 RM of 212 and you want to do 3 reps, you should be able to use 225. So I would probably go with 215 or 220 if I were you.