I don't know what you mean by 3g/s cable and third primary master and third slave. However I can tell you that there are only two IDE connector on most modern motherboard and that is the Primary IDE and the Secondary IDE, the third is for the floppy (this one is a bit smaller than the two). The Primary IDE is where you should put the Harddrive and the Second IDE is where you should put your CD/DVD drive, although you can put the Harddrive and CD/DVD together on the Primary IDE, however you'll have to set the Harddrive jumpers as "master" and the CD/DVD as "slave". If done this way and correctly your BIOS will indicate that you have your Harddrive on the Primary Master IDE and your CD/DVD on the Primary Slave IDE. Leaving the Secondary IDE empty.
Primary Master =Harddrive
Primary Slave = Cd/DVD drive
Secondary Master = None
Secondary Slave = None
If you choose to put the Harddrive alone on the Primary IDE, you still have to set it as "master" (or cable select if it's alone). The CD/DVD drive can be put on the Secondary IDE and set to "master" (or cable select if it's alone). Then your BIOS will indicate
Primary Master =Harddrive
Primary Slave = None
Secondary Master = Cd/DVD drive
Secondary Slave = None