You can do what you want with raid. You can set up a raid 5 system with 3(or more) disks, and have more than 1 drive of total space, and redundancy in case 1 drive fails, and can increase performance. The system will see 1 volume spread out over several disks.
If you have 3 120GB drives, you'll end up with about 240GB of usable space in a raid 5 configuration. Adding more drives will increase usable space.