that question is unansewrable in all cases.
What you'll need to do is to buy a suit, and then do a bouyancy check just like you should have with your 3 mil.
My guess is that an additional 12-15 pounds would be sufficient, but I'm curious - how tall/how much do you weigh, and what is your true waist measurement?
I'm 5'8", I weight 220 and I dive a 6-lb backplate/wing/24 pounds with a trilam drysuit and 300 weight undergarment. AND I'm probably a pound or two heavy with a 100 cf aluminum (but the swing weight of the tank full to empty is around six pounds, I figure). I'm not morbidly obese, but I consider myself pretty chunky.
I would recommend a trilam and then do the bouyancy check. There's no way to do anything but guess, and my guess is that you're pretty overweighted in the 3 mil already.