I would go with a Raid0 or 1 simply because RAID5 has a decent CPU overhead to deal with. They will still see the 'transferring files' message because that is dependent on your external media, and everything from eSATA, gigabit, USB, firewire, etc, is going to be significantly slower than a single SATA drive, much less one in RAID.
Just wondering, but what laptop did you find with 3HDD slots?? Must be a monster!
correction; RAID5 has a CPU overhead (or at least a much larger one compared to RAID1 or 0) if it is software RAID. If the RAID controller does the math then my answer would be RAID5 all day long, but I find it rather unlikely that a laptop would have a pro grade RAID controller in it.
Seeing this now, the mini card is a sata 3gb, thats a big deal. Raid 0 on sata 6gb would probably crush it in performance. However, I don't know if I want to risk that high of a failure probability. Anyone else use Raid-0 on a laptop?