In 1980 Anderson managed to make it into the debate against Reagan and Carter.
Reagan wins the primary in 1976 but goes on to lose. Ford or Bush is the 1980 nominee and conservatives aren't sold, so they run their own conservative candidate (Reagan? James Buckley?)
Bush/Ford, Carter, Anderson, and Reagan/Buckley all end up part of the 1980 debate.
Alternatively you could swap out an independent conservative ticket for a Libertarian one (Paul-Koch, Paul-McCarthy, somebody else like Mike Gravel, idk).
------------
In 2000 John McCain was offered the Reform Party nomination but said he wasn't for it. In three-way polling he was at 24%.
Gore runs further to the center out of concern that McCain will pull centrists from him, ultimately alienating more liberals.
Nader gets Congresswoman Cynthia McKinney to be his running mate, resulting in Nader polling more strongly come September.
When McCain is invited to the debates, the Bush campaign calls for Nader's inclusion and McCain joins the call. Gore, not wanting to look bad, concedes. The debate is thus Bush, Gore, McCain, and Nader.