$!*@, I totally forgot about Antonioni's trilogy, or Van Sant. But I guess, one could say that they are not really trilogies. 

Fortunately we still have the "other" option. As it is, I cannot edit the poll after it has been added. 

Haven't seen a lot of them myself (usually just one or two films). 

@wpqx 

I didn't vote for Back to the future, but it could be the same. I'll try and hold off till I see more trilogies.