Its possible to keep them in the same tank but there will likely be fighting and injuries amongst the crayfish and crabs (they are most vulnerable when they molt, unless the tank is large and there are plenty of hiding spaces for all of them). The snails may also get eaten by them too.
Do note that rainbow crabs and most of the other small crabs available at LFS are semi-aquatic and spend time out of water, so they need accessible land areas... compared to crayfish which can live in fully aquatic environments. So you'll have to setup your tank accordingly.
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