I've used 3, or 5 with no problem in Puget Sound at ~50F water temps. For me, the glove flexibility is less important than the method & glove material. I start by approaching slowly and then push them down into the sand so they can't run. At this point you have plenty of time to get a good grip on the carapace and underside. It helps here to have some good rubber grip surfaces on the glove. I try to put my thum on the bottom to get the most grip without getting finger too close to the claws. Now it's easy to flip the crab, sex and measure. Also, if I have 5mm gloves on, I can barely feel the claws grab, for the 3mm, have to be a little more careful.