HI Shannon,
Definatly keep these frogs separate as they secret toxins that are harmful to each other. Green and Golden Bell tree frogs are fairly easy to care for. I dont keep Gray Tree frogs but they sound like they have simular requirements. Read the care articles. Green and Golden Bell's like to sit up high during the day and climb around on the ground in the evenings. I suggest a half land half water set up as they love to soak in the water. Put plenty of plants and branches to climb around on and it also makes them feel safe and are more likely to sit in view. I also suggest a variety of foods and be sure to dust the food with calcium/vitamin D supplement.
Here is my Green and Golden Bell frogs enclosure.