What is the specific reason to use three disks in a raid5 and a separate disk? Why not a four disk raid5?
Assuming a single disk has a sustained transfer rate of 120 MB/s, your raid5 will deliver - depending on your raid controller - around 200 MB/s. However if you were to use a four disk raid5, that would increase to around 300 MB/s.
Currently your fastest disk is the raid5, so from a performance POV that would be the ideal disk to store your page file, previews and media cache files on, but from a storage efficiency POV, the single disk would be more attractive.
My initial reaction, without knowing your arguments for the 3 + 1 configuration, would be to use a four disk raid5 array and store everything, page file, previews and media cache next to your project and media on the same array. Easy to configure and maintain.
It sounds like OP is using the 3 disk raid5 as his OS as well.
Well he should then be using the one drive for his C: drive and the three drive RAID 5 as the project disk rather than bass ackward.