Absolutely NO-ONE despite some people thinking there has been. Trust me, I've been hearing this asked here and on IMDb again and again, and if they say no-one over there, believe me, that is the correct answer.
It's just one of those questions that do the rounds for some stupid reason.
Christopher Lee has come close.He was in Lord Of The Rings,The 2 Towers & The Return Of The King.He also starred in The Three Musketeers(1973),The Four Musketeers(1974) & The Return of the Musketeers( 1989).He played the same character in these movies.
Eddie Murphy is another 1 that has also come close.Beverly Hills Cop 1-3,Shrek 1-3 (Shrek 3 comes out next year).However he only made 2 Nutty Proffesor & 2 Dr Doolittle films.
harrison ford has also done 2, indiana jones and star wars and is very close to 3 as he has played Jack Ryan in Clear and Present Danger and Patriot Games.
Technically, quite a few people have appeared in three film trilogies. However, I assume you mean who has appeared in all three films in three film trilogies, to which the answer is no-one (certainly not Sarah Michelle Gellar who doesn't even come close- she hasn't even appeared in all three films in one trilogy, let alone three)