It is very difficult to ID sex when younger and still hard when older. You will likely see them pair up and they get older (you have a good change of having one of each sex with 3) and start hanging around more as a pair. Then you will see the male likely fighting to the other male if it is a male, other wise with 2 F, may not be as much fighting.
You will eventually see a female with a tube to lay eggs and a male will follow behide and fertilize the eggs. Then you will for sure know. By behavior you can often get a good idea which is which.