> That only works if the message of the third party is more appealing to those voters.

It actually works just as well if the third party fails to attract the voters with its message but provides a reason not to vote for the targeted major party candidate that would not work as well if the messenger was the major party using the third party as a stalking horse. Because discouraging voters that would otherwise vote for the other party has the exact same effect on the outcome as moving them to a minor party.