I just returned yesterday, let me first say that this was my first trip to CR, and so I have no idea if conditions are different in August. Perhaps someone with more experience can answer better.
FWIW, three mill should be fine, with a hood along just in case. From my experience, you usually run into at least one thermocline each dive, though never for too long.
I would definately bring gloves along as well, though I try never :58: to touch the reef, Costa Rica doesn't have much of a reef, and there are plenty of bare spots to put a finger down if you have to, that said, several people cut their fingers because they weren't wearing gloves.(even the dive masters would plant a finger every now and then)
However, :banana: LOL, everything is constantly changing, surge, vis, and temperature...
So your mileage may vary, just enjoy the adventure, though not much of a reef, there is a pile of life down there! Have fun, LOL, I am ready to go back.
Ken