There's no catch, they want you to distribute it to as many people as possible. I ordered 10 copies of each of the first distro and managed to convert six people to Linux so I consider that operation a success.
Or just download whichever distro you like for free from www.linuxiso.org or any one of many other places.
You DO have a choice.
OK, 3 x 700MB are three big downloads, but if you have broadband, each CD takes about 3 hours on average, start the D/L before you go to bed for three nights running, use checksum, then burn 3 x ISO's to 3 x CD's.