The USB partition is a back up for my USB stick which I use for school. It is only 30GB big so not big enough for much. My friend is going to install the OS onto the Ssd for me so I can just plug it in and go.
I would really not recommend doing this. The OS installation should occur with the drive installed in the system it is intended to be run in. Otherwise, you don't get a clean install at all. Drivers and supporting settings for the system it was in when he installed the OS will be there when you plug it into your system, at which time windows will realize it's in a new system now and attempt to find and install drivers suitable for your hardware.
The problem with this is that sometimes drivers are left intact or partially broken, registry settings become conflicted as references to the old hardware fail to be removed and just in general the whole point of a clean install is sidelined. If it's at all possible, have your friend do the install while the drive is connected to your system or install yourself using his or her disk, or get an installation disk of your own.
Yes, there is only one physical drive but split into three parts. Would it be best to wipe the entire HDD and just sort of start a new or can I just wipe the OS partition.
If you have a third party solution for transferring the files off the current hard drive, reformatting it and then transferring them back on, that's great. If not, you can just delete the OS partition and incorporate it into your game partition, leaving the USB partition alone if you wish. Or you can do the same with that partition as well. It's really up to you.