Well, if you want to use reported appearances in films, then go with...
Julia Louis-Dreyfuss (National Lampoon's Christmas Vacation) Johnny Galecki (Suicide Kings) Henry Thomas (E.T.) Harrison Ford
Harry supposedly played the faceless teacher in Elliott's science classroom. But since it's unconfirmed on IMDb...
Julia Louis-Dreyfuss (National Lampoon's Christmas Vacation) Johnny Galecki (Suicide Kings) Henry Thomas (Legends of the Fall) Brad Pitt (The Devil's Own) Harrison Ford
I'd continue the Seinfeld trend with Wayne Knight but that's just too darn easy (Jurassic Park - SLJ), so how about Jason Alexander? Should still be easy.