I'm sorry your frog is not doing well

A 12x12x18 is way to small for three whites. Even babies. For three a 24x18x24 is the minimum

A mealworm or two once in a while won't hurt, but keep in mind they have a tough exoskeleton and minimal nutrition. I would just not waste time money and energy feeding them mealworms

Moss is always an impaction risk. Recommend remove it.

Frogs are solitary animals by nature. They tolerate each other and will not get "lonely".

Color change is completely normal for a white's. Don't worry about it.

As for the bloat, it may be a calcium deficiency. Make sure you use both calcium and vitamins. Cal+d3 every other feeding and vitamins once a week. I recommend the powder over the liquid.

And i also would take the frog to an experienced amphibian vet or at least shoot one an email.

Do not put the frog back yet at least for a few weeks. Don't want to add any unnecessary stress