To me, a three-watch collection is about versatility. If I’m spending five thousand bucks on timepieces, I need to have all my bases covered: they’ve each got to be watches that look and feel good with a tux and with a tee, in the woods or at the bar. Of course, I’m not going to buy three watches that do exactly the same thing, either. I wear a watch as an adventuring tool, and to look and feel good every day, and to dress up nice when I have to. Covering these bases also helps me pick three watches that are distinct enough from each other to not want to head over to /r/watchexchange every few months.
The other fun test I like to run in my head for a watch I’m thinking of buying goes something like this: Fifty years from now, if my kid or grandkid were to find one of these watches in my desk drawer, would they make me look cool? I think each of these three will make them think Grandpa was a real cool cat, regardless of the truth.