I suggest to wait untill it´s fully grown... This for at least two reasons. At first when it´s fully grown you´re sure that it´s a female, in case it turns out later on to be a male you have a problem when you put it in the same viv as the adult male.
Secondly, when you put a smaller animal in a viv with two bigger ones, it will be dominated strongly, and will probably not get all the food it needs, and will not grow up very well.
So if I were you, I would wait untill it´s fully grown. I actually wouldn´t put more than one female in a viv, looking at the territoriality of the species, but that´s something else...