RS National Dex TAS explanation

As of now, it's not possible to create a gen 3 National Dex TAS. Ruby, Sapphire, FireRed, LeafGreen, and Emerald will not be able to complete the National Dex for all of the games, as Ruby and Sapphire both require Ho-oh and Lugia, which are only obtainable through the Game Cube games. (Ironically, the other games do not require Ho-oh and Lugia). Even if this limitation didn't exist, the current emulator for tasing (Gbahawk) only allows four games to link up instead of five. FR, LG and Emerald are not enough to complete the National Dex as Emerald is missing a few Hoenn pokes (Lunatone, which is Sapphire exclusive and Zangoose, which is Ruby exclusive).

With a gamecube game in XD, it would be possible as it has all of the missing Hoenn mons from Emerald, thus eliminating the need for Ruby/Sapphire. As it stands now, it's not possible for a tas to trade between GBA and Game Cube (because they're two completely different game consoles lol) but assuming that an emulator would be made to allow for that, it would be doable. We'd need four different games and there are two different combinations that allow for the full National Dex: FR/LG/Emerald/XD, and Colo/LG/Emerald/XD. Ironically, LG/Emerald/XD get every single poke except Qwilfish, Murkrow and Delibird

Either combo would require Emerald to complete the game twice and reset, in order to get both Latias and Latios. It would also require Emerald to do an early reset to get the third Hoenn starter.

The Colo/LG/Emerald/XD combo has the advantage of easy access to both the Johto starters, and legendary beasts, as it can get all of them in one save file. However, having only one Kanto game requires LG to complete the game and the Sevii islands quest three times for the Kanto starters, as trading with any other non Kanto game is prohibited until then.

The FR/LG/Emerald/XD combo eliminates this problem as FR and LG can trade in between each other whenever they'd please. However, they'd still need one of the games to do the entire game + sevii islands quest twice for the legendary beasts, as those are tied to whatever starter you pick (Raikou to Squirtle, Entei to Bulbasaur, and Suicune to Charmander) and you can't get more than one per save file w/o trading. The bigger issue tho, is the Johto starters, as w/o Colo, the only ways to get the Johto starters is by either completing the hoenn dex in Emerald, or completing Mt Battle in XD. One of Emerald or XD would need to do their respective task twice in order for everyone to get all three johto starters. The Mt Battle option is def faster but it would still probably take well over 4 hours for one run.

Of these options, the Colo option is almost certainly faster but it's still an immense amount of work, at least 5 times more than even the gen 2 diploma. Even if the technical limitations weren't a thing and it was actually possible, I would still be hesitant to do it but it would certainly be a possibility.