Like Debby says, the weather is a crap shoot, but then again, it is a crap shoot any time of year. At least you shouldn't expect rain in the winter, but the winds do blow, and the seas can be rougher in the winter than in the summer.
The viz is better in the winter than in the summer IMO.
Would you need more than a 3mm is a tough question to answer. IMO yes. I would recommend at least a 5mm and maybe a hood. Last winter, some friends from Colorado came down, and they were very happy that I recommended the 7mm suits to them, especially since we did 18 dives in 5 days... Personally, I dive dry in the winter so there you have it.