I would leave three. Many would tell you to cut down to two or even one, but there are so many things that can happen to melons before they produce, that I like the insurance of three. Once two of them have started to form melons, you can cut the third or smallest vine if it seems that they will become over crowded. With pots that small, you may need to water every day once they get going. Good luck.

I agree with ohiofem, just make sure you cut the right one out when that time comes :) Also, make sure you are extra diligent with staying up on the watering when they are in containers, if you give them too much water in one shot after not watering for a short period of time, you may risk the splitting the melons.

I grew some crimson sweets last year in containers smaller than yours, and I got a few ripe melons, but they were far smaller than they should have been, probably due to crowding in the root zone. A couple also split because if forgot to water one day and then saturated them the next, a couple of the melons split within hours.... they were ripe though (or close at least) and I discovered them soon so I ate them anyway:)

Thanks for the feedback you guys,I appreciate it:)