Landlord sold property to 3rd party while he had a Lease Purchase Option with tenant. Now what?

It depends. If the deposit money or from rent was supposed to go to an option then it could be enforceable via specific performance. See a lawyer to go over the exact facts

If the option to purchase is valid and tenant has otherwise complied with terms and not waived its rights, a demand can be made by tenant on the 3rd party to sell the property to tenant under the original terms under the doctrine of specific performance. The remedies of the 3rd party, if any will be against the landlord / Seller or under any title insurance policy. See an experienced real estate lawyer.

I agree with Nestor Gorfinkel. You may have enforceable rights against the third-party purchaser, but you need to consult with an attorney and have all of the pertinent documents available for review at that time.