What damages can I sue for if a previous employer trying to enforce an invalid non-compete clause?

A 36-month noncompete is certainly unreasonable--that's much longer than senior executives are usually held to when a company is sold and the buyer doesn't want the old management competing with him. Whether a state-wide one is reasonable is industry and job specific--it's probably too wide, but not definitely, not if in your line of work someone can effectively sell statewide.

Given that it is so overbroad in terms of time, it's almost certainly nonenforceable. Given how unreasonable it is on its face, it may then have been unreasonable for the old boss to call the new employer and threaten based on it.

You may have a cause of action against your old boss, and you should definitely be able to get the noncompete either struck down or at least limited to reasonable terms. (Some courts will do that, instead of invalidating them completely.) It is definitely worth your consulting with an employment attorney. Good luck.