I think there is your problem. 10 gallons is just too small for 3 frogs. You are probably suffering from water quality issues. Would you be able to upgrade to a 20G Long? They are not very expensive and would house 3 frogs but I would not add another.

Also what is your filtration? You'll want at least 2x filtration (so 40g of filtration on a 20g tank if not more).

Do you test your water? ACF are sensitive to water quality just like fish. Ammonia/Nitrite/Nitrate matters, do you have a test kit?