First of all, the third generation games were Ruby, Sapphire, and Emerald.
Second of all, Yellow was a part of the first generation: Red, Blue, and Green, which was remade as Fire Red and Leaf Green for the Gameboy Advance.
Crystal was a part of the second generation: Gold and Silver, which was remade as Heart Gold and Soul Silver which came out a little over a year ago.
Platinum was a part of the fourth generation, along with Diamond and Pearl.
l HIGHLY doubt they'd make a remake of these games, since their so new already!
As for the third generation (Ruby, Sapphire, and Emerald), there has been speculation of remakes in the future, or possibly a third fifth generation game.
(+) Link made me realize the true question that you asked.
The Pokemon company and Gamefreak probably don't want to make remakes of each third game in every generation, because that would make them less money.
Notice that every remake (Fire Red, Leaf Green, Heart Gold, Soul Silver) are doubles, which forces you to either trade, or buy both games in order to get certain Pokemon, and to complete the Pokedex.
Many other games have done this, such as an old GBA game, the Megaman Battle Network series.
I hope l clarified well. ~