That's odd. You're getting 600 GB (300+200+100) from a 500 GB drive? ;-)
You need to be careful if you are installing anything but 64 bit W7, you need to worry about how the real GPT partitions are seen as MBR by the BIOS emulation required to boot Windows.
I was just saying on another thread that I didn't recommend a 3 partition setup, it's just not worth it IMHO.
Yes you can certainly do this, but with a little tweaking. You still need the bootcamp bootloader so that option works after the installation, otherwise it will only boot to mac os x.