Generation 3 featured the first games on the Game Boy Advance which meant a lack of backwards compatibility compared to Gens 1 and 2. That also meant a soft reboot with Ruby and Sapphire and a lot of controversy since you could only get the new Pokémon in that game… until Game Freak remade Red and Green into FireRed and LeafGreen. But even then, catchin’ ’em all was a near impossible task back then and almost certainly now (without cheating). So what to do? Well, Lyra made a website addressed all of these constraints and controversies in the above video and it’s well worth your time if you never experienced those issues or if you need validation of your living dex struggles. And yes, they’ll will be making a living dex in Gen 3. Good luck to them.
On a similar but more angry vibe (with swearing), check out this Reddit rant about completing the Gen 3 Pokédex.