What? Your question doesn't make sense. You have three OSes installed on two hard drives within three partitions? For questions about formatting or erasing OSes, you want to be precise...
I assume you want to delete an OS or two and use that drive (or partition) to store files on. Boot from one of the OSes you want to keep, copy the stuff from the partition you want to delete, then open DiskUtility (in Applications/Utilities/) and select the partition you want to wipte and erase it. Make sure you pick the partition and not the drive.