3 feet is a bit iffy for most reptiles.
4ft is needed for larger lizards like beardies and blueys.
I'd go for nothing larger than 2ft for geckos.
Since it can hold water, maybe get some yabbies or crabs?
Thought about a turtle? If the pumps/filters are still good it'd be great to start off a turtle.
Frogs? I dunno if you can really keep them in a tank, though...