I thought about this after recently posting about John Ritter appearing on the series finale of Mannix:
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Granted, this was before Three's Company and John Ritter wasn't exactly a household name yet, but it's still extremely bizarre to see him play some type of tough guy villain given his affable nice guy image in real life and even on Three's Company.

The second "weird" role for a Three's Company cast-member to come to mind, is Priscilla Barnes in the second Timothy Dalton James Bond movie, Licence to Kill. Priscilla in it, played the doomed wife of James Bond's colleague from the CIA, Felix Leiter. I suppose that after immediately recognizing Priscilla from such a lightweight show like Three's Company, it's a big shell-shock to see her in such a dark and violent movie (by James Bond movie standards), where her character doesn't make it out okay (https://www.youtube.com/watch?v=1LcUBRQ_JFM).

There's the "John Ritter Comedy Special" that features John as various characters although we still can see a bit of Jack Tripper in some of them. Joyce also did a sketch.

https://www.youtube.com/watch?v=CGNwRFVeCoM

I once wrote about John Ritter appearing as himself on Life with Lucy. Nothing wrong about John playing himself, except he played a very clumsy Jack Tripper in the hardware store; tripping over things, bumping into things and goofing off on the rolling wall ladder. They should have focused on Jack Tripper opening another bistro in Lucy's neighborhood. I would like to think the real John Ritter can walk down the street without being a human catastrophe.

Oh nice Brady Bunch staircase in that Mannix scene.