The mystery tadpole died a few days after posting this. It had abrasions on it that I noticed when I introduced it to the Planet Frog. Possibly it had tried to bury under the gravel?

My two remaining froglets are now sharing a 40 gallon breeder with expanded cocofiber bedding, water-washed-and-baked magnolia leaves, and a purchased water dish with a smooth ramp they can use to enter and exit their "pool." One likes to hide under the leaves, and the other half-buries in the bedding when it isn't in the water. I still intend to turn it into a half-water, half-land set-up, vivarium style, with a false bottom, land and water plants, and pill bugs and springtails on clean-up duty, but I'm not there yet.