If I signed a contract that stated " all sales final", can I still invoke the 3 day right of rescission?

It depends what the contract was for. Typically, in most states, this right to rescind applies to door-to-door soliciations, gym memberships and the mortgages. Otherwise for most other consumer sales (cars, leases, et al), there is no such right. To be sure of the law in your state you can cntact your state's department of consumer affairs or the attorney general's office.