It's supposed to take 3 parts. I told Allie a few months ago that you only needed to wear 1 to get the bonus. Being the level-headed Admin she is, she promptly told me I was wrong, refused to believe that only one Tier 3 part could give the bonus, and I went on my merry way to abuse this opening she had left in the game that she wouldn't believe existed.
This isn't really a bug, since you're SUPPOSED to wear a full set to get the bonus. It's a bug within a bug. Now that you've made the thread with photo evidence I'm sure Kenny/Allie will patch it up to requiring all 3 pieces for the bonus. Doesn't bother me but that's gonna eternally piss off half the game which are by now used to only needing one tier part for the bonus.