If you think 3 may not be enough, but 5 may be too much, why not just go with something like a Neosport 4/5 suit
Or the other option is just do the 3 mil, then get a Henderson Sleeveless Shorty to wear over it if you get cold. That would then turn your 3 mil into a 6/3 suit. And it give you the added option of wearing it just as a shorty if it's really warm.
Or the other option is just do the 3 mil, then get a Henderson Sleeveless Shorty to wear over it if you get cold. That would then turn your 3 mil into a 6/3 suit. And it give you the added option of wearing it just as a shorty if it's really warm.