It's a matter of opinion, sure. That doesn't make it invalid though. Especially since I took many opinions into account when writing that statement and I didn't say all trilogies are bad. I said "sometimes you should stop at 2." Believe it or not, I enjoy Return of the Jedi. However, I wouldn't say it was the high-water mark of the franchise....most fans wouldn't.
Movies may be an unfair comparison, so I'll stick to games. To be fair, I do enjoy a good 3. I think Uncharted 3: Drake's Deception is better than the last. Critically and financially speaking, it faired better than its predecessor. I'd go so far as to call that a certified hit, while I'm sure not everyone enjoys it. Still, its success is undeniable. Grand Theft Auto III is stellar in many respects. I can't say the same about Twisted Metal III, or Silent Hill 3, or Resident Evil 3, or Tomb Raider 3. The last few may have recieved some praise here and there, but they're not held in the same high regard as their previous entries, nor did they sell as well. Not really. I'm not saying they're terrible; just not as good.