Sooooo you currently can play 3 (4 if you count VC gen 1 but you already said that you want to pick up a phisical version of R/B so it doesn't count) different generations on the 3DS: 4th, 5th and 6th, If you buy a GBA you should be able to play a game from each generation, personally I feel like since the Pokémon franchise is so atached to the concept of Evolution that it's the best thing to do to just play them one after the other. of course instead of purchasing all three the versions for each generation pick up the third one for all of them, excluding Gen 5 where since there were 2 sequels you should pick up both one of the first two (BW) and one of the latter two (B2W2).