Based on my experience you could add the laptop drive and not notice a loss in speed depending on what you're using the storage for.
Say you have 2x matching model number drives (3.5in, 7200rpm) and 1x laptop drive (2.5in, maybe 5400rpm). Striping data across all three drives will make the array run at MOST 5400rpm but it'll likely be a bit less for overhead. This being said you'd normally pick a RAID0 for speed or total capacity so adding the third drive would make sense if you wanted the capacity more than the speed. With a 120GB SSD cache I would toss the laptop drive in there because you'll likely only interface with the SSD as it's caching capability will likely make it so you never see the array's real performance. Unless you copy over 120GB before it clears to the array.
The failure point is worth noting. See previous statement about what you're using storage for to make your own decision.
Striping means no parity or backup, if one drive dies they all die. I like HD Sentinel for checking drive health and I've had one 2.5in drive actually fail in my experience (Seagate if you're wondering). Fortunately it was a 1TB RAID1 for developing VMs so no data was lost but the rebuild took the better part of a day.
Hope this helps, honestly if you're doing nightly backups and not doing anything super critical on it I'd do the three drives together. Sounds like a good setup for a LAN cache maybe. Cheers.