I think that what you are asking is not even feasible, at least for my knowledge.
Those two kinds of disks will most likely have different connectors, not only but because of their different voltage, but also transfer speed. I think phisically speaking you may not be able to connect all of that 3 disks in the same RAID 5 array…
What you are saying about transfer rate is completely right.
Suppose you are going to do a thing more feasible (and that will work) like joining 3 SCSI disks to a RAID 5 array. Despite the brand, those disks will be 2 of 10K Rpm and 1 of 15K Rpm.
The whole array will operate at the lowest common denominator, which is 10K RPM.
The same applies to the capacity. Considering RAID 5, If you take 3 disks, being 2 of 73 GB and 1 of 146GB, this one disk of 146 GB will be just half used.
The total available space in this scenario will be ~146 GB (in RAID 5 1/3 is reserved for distributed parity).
Let me know if you need some more clarification.