Welcome to the forum.
The 12x12x18 is too small for the Agalychnis callidryas (Red Eyed Tree Frog) and if your thinking on three, then your best bet is the 18x18x24 Exo-Terra or the Zoo Med.
I think the Exo-Terra is nice and like the door latch better than the one on the Zoo Med.
I have two and they are in a 18x18x18 Exo-Terra and I wish sometimes I opted for the taller one since it was the same price.
Red Eyes need height and plenty of leaf/vine travel area since they will rarely walk on the substrate. Low to high plants in the viv do these frogs good and large leaf plants give them plenty of night time perches and places to feel secure and hide.
Go for the bigger Exo-Terra and you and the frogs will be happier in the long run.