When counting the third versions, you'll have to consider that a non-insignificant amount of sales will be to players who are double, triple or even quadruple dipping, so even if total sales per generation remain similar, the actual playerbase is likely smaller for generations with third versions. Additionally, we can look at the sales of the remakes, which show a clear upward trend.
Firered/Leafgreen: 12.00 M
Heartgold/Soulsilver: 12.72 M
Omega Ruby/Alpha Sapphire: 14.60 M
Let's Go Pikachu/Eevee: 15.07 M
Brilliant Diamond/Shining Pearl: 15.06 M
Bonus: Legends Arceus, which is considered part of the main series, has sold 14.83 M copies. This is the most any single Pokémon game has sold, and since it's completely stand-alone, there are no double-dipping shenanigans going on here.