This may not be as troublesome as you think. I am assuming that you're going to use the mobo's built-in RAID0 management system. On machines like that SOME allow you to set the mode of each SATA port separately, but some treat them all the same, so you might find you have to set ALL SATA ports to RAID mode in order to create and use the RAID0 array. HOWEVER, once that BIOS setting is made then you must enter the RAID management utilities to set up the RAID array. You will find within that system that, by default, NO drive unit actually is used as a RAID component unless you assign it to a particular array. So you should find that you can assign your two new WD blacks to the RAID0 array and set them up, but the other older HDD will NOT be used as a RAID device unless you tell it to do so.
I suggest you do all your setups with the older drive NOT connected to the system. Once it's working, connect up the older drive unit and let it discover a new device on a SATA port. At first, just do NOT go into the RAID management screens and let it default to being a non-RAID device. It may just work fine, as long as the OS can handle this unit in terms of drivers. Although you had it installed in a previous system with its port set to an IDE Emulation mode, that does not affect the way its data was written. So in your new system, even if the machine defaults to treating it as an AHCI port mode it should be completely readable IF your new machine's OS has AHCI drivers built in (Vista and Win 7 do, XP does not). At worst you may have to use either the BIOS Setup screens or the RAID management screens to set the port mode on this older drive to IDE Emulation again if your new OS can't handle it as an AHCI device. OR, better yet, since the old drive will be a data device only (not a boot disk), just install the normal AHCI device driver in Windows so it can use the drive in that port mode.