I took a look inside the computer and found this at the other end of the two SATA "combo" connectors (one of them is connected to the DVD-drive, the other connected to an SSD):
- on the motherboard (the other end of the two "combo" cables):
- two SATA 7-pin right-angled connectors
- one small square 4-pin power connector ("PCIe power connector" I believe it's called, or is it an "ATX power connector"?)

There are also two power cables from the motherboard connected to the ATI 5870 graphic card. The cable ends that go to the graphic card:
- two big 6-pin squared power connectors (called a "PCIe power connector" as well?)
Having given it a little thought I'm a little hesitant about using those combo power adapters as they're bulky and can easily loosen when mounting the drives, so I agree with you guys not to use the "combo" cables if I can avoid it.
I figure I can simply replace the data part of the "combo" cables with one 7-pin SATA cable for each drive.
The challenge is supplying power to all 3 drives

I can see one of two options, if any such adapters/cables exist:
- a 4-pin PCIe power connector at one end of a long (approx 50cm) cable with 3x SATA 15-pin power connectors at the other end (the 3 drives are right next to each other)
- a 6-pin PCIe power connector/adapter which plugs into the graphic card as before, but splits into an additional long (approx 50cm) cable which goes to the SATA drives and ends up as 3 SATA 15-pin power connectors.
Is anything resembling one of those available?