I have both. The 3mm is easier to stretch so if you are right at the edge on the 3mm you are probably taking advantage of the stretchiness. Just due to the nature of a 7mm being so much thicker is is going to be a harder stretch. You may be a bit tight. 7mm is a bit of a chore to get into so it's hard to tell you. I just went up and did a non scientific pull test on each suit. They are both stretchy, the 7 slightly less but not much. I did find the dimensions to be the same between them. I think what it comes down to will be how much you are pushing the size due to stretch.