I also had the problem of partition 3 not showing up (I tried the HEX editing but the values where already there).
The solution for me was to play a game on the 360 and create a savegame on the new drive. After that I could see a partition 3 in Xplorer360 on my pc. Maybe a partition 3 only gets added by the xbox360 when needed instead of automatically when formatting the drive?

Grtz,
BParty