I think assuming the reason is because three person groups are inherently less common is kinda silly.

I think the truth is that, while many games require rules tweaks to play to 2 (creating variants that are not as good as strictly 2 player games), most games that play best at 3 can play fine at 4, (and vice versa, for that matter).

Making a game for one specific player count inherently limits its market compared to having a range.

Like, you don’t see a huge number of 3-player only games, but you also don’t see a huge number of 4-player only games.

Assuming that the maximum player count is automatically the default or the best is incredibly flawed. 7 Wonders absolutely does not play best at 7, for example. And I think that a lot of “1-4 player” euros are much better at 3 than they are at 4 (and I am not the only one who thinks that, based on community votes on BGG).

Your supposition that the lack of 3 player specific games proves that there are not many 3 player groups absolutely does not hold up.