The Game Cube SDK is not free, regardless of how good you are, last time I checked it was around 10 grand to get started, and the restrictions as to who is allowed to use it and for what are very, very tough.
Considering you have all three consoles, if you want to make games for them, either hack and download Microsoft's SDK illegally (Maybe if your game is really good they'll overlook it
OR
much easier AND legal, the PS2 comes with its own programming language anyways, its on the demo CD you get when you buy the PS2. You'll need a USB keyboard and lots of space on the memory card to save, but decent 2d AND 3d graphics are possible, and you have a good chance of getting paid for your game to be distributed on the official PS2 magazine. Also if your PS2 is online, there are many resources for PS2 programmers using this method.
Personally, whilst I don't really like a lot of decisions Sony made with PS2, I think it was exceedingly generous and forward thinking of Sony to make this opportunity available to all PS2 owners for FREEeeeee!
Also, the Yaroze kits are still available and they are pretty darned cool too
Quikly Studio Pro. Soon. Honest.