It's really easy to think of examples that debunk this post's thesis - Morrowind, Super Metroid, Tekken 3, etc etc - but sometimes this post does ring true in the industry; some third titles / threequels are, in fact, the absolute worst of their trilogy
ALT Doom 3
ALT Watch Dogs Legion
ALT Payday 3
ALT Postal 3
Valve is probably right for never making third games. I can't think of a single third game that was better than the previous two.