That is one very strange cable, i have never heard of a Sata cable that puts power and data into the same cable, nor a sata Data cable with split ends. I have always known data cables to be 1 end to 1 end.
Can you post a link of your cable? Or does it look like this one:
http://www.svc.com/mod-4p3sata-bk.html
Re-reading what you have said about the Sata cables, if it is a power connector on one end, and 3 sata connectors on the other, then it is definitely a SATA
power cable, not a SATA Data cable. The sata power port is wider than the sata data cable (you can compare the size of the port to the SATA ports on your mobo, if the cable has wider ports, it's definitely a sata power cable.
It is quite strange that motherboard would supply a Sata power cable rather than sata data cables though.
EDIT: What model is your mobo?