Hi Scott and welcome to Frog Forum
The first thing you need to do is to get the frog in an aquarium. These frogs demand clean water for them to thrive. Since you already have two in your 10 gallon tank, it shouldn't be a problem to add one more. You may want to monitor the behavior of the frogs periodically to see if there any behavior changes. I don't think age will be a factor. Good luck!