I usually buy one and then the third game when that one comes out, and the one that I buy usually alternates between the 'primary' game and the 'secondary' game. The only exception to this is the first Generation in which I only owned Pokemon Yellow.
Then there was Pokemon Silver which is the secondary game. I didn't own Crystal until much later, but I still got it.
Then I had Pokemon Ruby which is the primary game, followed swiftly by Emerald.
And then it was Pearl (secondary) and then Platinum.
And then Black (primary). There was no third edition to GenV, although I intend on getting Black 2 once it has its English release.
Interestingly enough, my cousin always buys the game that I didn't. She had Gold, Saphire, Diamond and White. None of the third editions though which is a shame. They're usually much better than the original two since a lot of the game is revised and improved.