Replacing a hard drive in a RAID array

I have a question about RAID arrays – For example, you have a RAID array with 3 exact duplicate (same everything) 9GB SCSI hard drives that were set up three years ago. If one of the 9GB hard drives in the RAID array dies, what happens if you can?t replace it with an exact duplicate make, model, size, etc.? Can you add a different make and/or a larger hard drive to the array or would you have to replace all three drives?
Thanks-
Glenn Hudgins