Mixing Dart Frogs?

[SerpentLust]

I'm very new to the amphibian hobbyist community. Sticking mainly to snakes and such for a while.

I saw little poison dart frogs at the expo and this was the second time that I was really compelled to buy them.

My question is, how many dart frogs can you keep in one enclosure? For arguments sake, we'll say a 3 foot long enclosure.

And my second question is whether or not you can mix them. Like if I wanted a Dendrobates azureus (Blue Dart Frog), a Dendrobates auratus (Turquoise) and a Dendrobates pumilio, just for example. Would they be allowed in the same enclosure? Or would I have to stick to one of them like a group of three Dendrobates azureus?

Thanks in advance,
Jenn