If you installed the BT5 as the third OS, this is, the last OS you installed was BT5 (W7, Ubuntu and BT5, in this order), no problem, download gparted live cd, boot with it, erase the Ubuntu partion, then resize the BT partition, and then reboot.
If you installed the W7, BT5 and then Ubuntu, in this order, do as mentioned before, only this time you will need to recover the grub. In this case you will also need a live cd from any linux distro: Debian, Ubuntu, LMDE, etc. and then recover the grub..It is easy..
Hope it helps.