Personally I would never mix any species,
All species have different toxins on their skin, toxins along with water and the minerals in the water are absorbed and released in the water they soak in, so if the 3 frogs are happy soaking together they could end up poisening each other,
Also Whites tree frogs are not fussy eaters, if its smaller than them and moves around its fair game to eat, this includes smaller frogs! Im having to seperate mine as I have 1 female getting rather large and fat rather quickly and shes already caught her boyfriends toe when going for food before! and my finger haha.
As for sexing the whites, virtually impossible until they reach 1-2 years, females tend to be bigger and the males like to sing for most of the n

Hope this helps