The problem may not be people leaving the Republicans to vote for a third party if Giuliani is nominated, but Rudy leaving to run or support a third party candidate. If he runs as an independent, or Bloomberg does and Rudy backs him, that could take a lot of votes from the Republican nominee.
Neither of those alternatives will happen. Guiliani certainly has no interest in any third party. Why would he? Only because of his love of country would he even CONSIDER running knowing the snakes lying in the grass ready to strike.