Anyway to cash a check post dated three days from now? What about a cash checking company?

In fact, a post-dated check can be immediately deposited or cashed. It's up to the financial institution whether or not it wishes to do so.

A post dated cheque can not be cashed until the day that it is valid.

Three days from now is monday..Why not wait for that date then cash it?

Well you might get someone to take it at a discount but probably not.

definitively not