T.R. ran third-party against his own handpicked successor, William Howard Taft, fatally splitting the Republican vote. Sad!

More recently, a similar thing happened when Bush the 1st & Ross Perot, split the moderate right vote, essentially handing the Clintons their first victory. Perot may or may not have been running to split the Republican vote, but he did have that effect as he pulled most of his support from the right.
As to whether Trump, DeSantis & the rest of the usual suspects will put aside their differences and run one right wing candidate (whoever that may be) will remain to be be seen. Two years is a long time.
But a third party trump campaign would probably result in a landslide Democrat victory as whatever third party votes trump got would reasonably speaking would be coming from the right.

"The 2024 presidential election is already over. Democrats won. Or rather, they will. Just ask New Jersey Governor Woodrow Wilson, beneficiary of his opposition’s inability to settle upon a single nominee in 1912. More accurately, and for the purpose of this example only, ask the Donald Trump of his age: Theodore Roosevelt. A former president from New York with inherited wealth, a loyal base of supporters, and an insatiable need to be the center of every story, Roosevelt’s refusal to step aside, even for a once-loyal successor who promised virtually the same agenda, broke the Republican Party’s lock on the electoral college.

"History is revving up for a repetition of that fateful election. For forty-four of the fifty-two years preceding Wilson’s tenure, and for another twelve after, the presidency belonged to the Grand Old Party. Roosevelt himself won the election of 1904 in a landslide, before handing his party’s standard four years later to his personal and ideological friend, William Howard Taft. TR left office the youngest former president in history—a distinction he still holds today—but couldn’t stay retired. More accurately he couldn’t stay out of the spotlight, and ran against Taft a mere four years later for the Republican nomination.

"Here is where the story gets particularly interesting for understanding 2024. Indeed, here is where it starts to sound familiar. Taft won the nomination, yet Roosevelt refused to admit defeat, claiming the entire nominating process was fixed from the start. 'Never has there been anything more scandalous' in American political history, TR railed with a sense of hyperbole wholly familiar to those of us who have lived through the Trump era. 'They are stealing the primary elections from us.' If his supporters, indeed the American people, wanted the country they deserved, the time for talking had passed. It was time instead to fight. 'We stand at Armageddon,' Roosevelt declared before leaving Republicans behind to forge his own party, 'and we battle for the lord.'"

https://www.msn.com/en-us/news/politics/the-2024-presidential-election-democrats-have-already-won/ar-AA148PlS?ocid=entnewsntp&cvid=3e61fdc67722410ea957b3285881c5d6