If I plan on wearing this over my 3mm wetsuit, should I order up 1 size to allow for better fit?
I would say no. This suit comes with super strech neoprene around the waist and arm area, so hopefully, getting it on over your 3mm won't be a huge problem. You just don't want the hood to be too big, or you will get water flooding in around your ears. Yikes! So I would stick with the size you would normally order, not one up.