To combine drives of different sizes into a single volume, use JBOD (Just a Bunch Of Disks). It can be done with Disk Utility, though you may be better off having them separate. If one drive in a software JBOD array goes down, you can lose all data on all the drives.
You could get three firewire bridge boards and mount the drives in your PC case. Thy should run off the PSU....
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