I have 3 primary partitions and 20 gigs of free unallocated space, so i am wanting to extend partition 3 which is NOT the root partition with the unused 20gigs. So part 3 + 20gigs, i have tried using diskpart, and a partition manager, diskpart doesn't seem to allow you to add the unused space to the part and the part manager seems only to allow you to shrink the part. Can i do this?

You could do it with windows. Might use free tools like gparted or live cd's that have it installed. Most live cd's have a partition editor. Be sure you consider backing up data before you attempt. "Best Practices", Event viewer, host file, perfmon, antivirus, anti-spyware, Live CD's, backups, are in my top 10