Check carefully in Disk Management. If your BIOS can recognize the third HDD, it certainly should show up in Disk Management.
In Disk Management, concentrate on the LOWER RIGHT pane, which shows all legit hardware drives, even if Windows can't recognize them yet. Then recognize that this pane (and the one above it) SCROLLS so you can see all it contents. You should be able to see boxes representing all three of your HDD's plus ones for your optical drive units. The third HDD might be the LAST in the list, depending on the history of its installation. If it it there but shows no letter name (like H
on its Partition, you can RIGHT-click on it and assign a letter that is not currently in use. It would usually be best to assign letter names the same way they were before, in case your Registry is expecting to find certain items on certain drives.
One other thing to check is the way the SATA Port Modes are set. (I am assuming that the HDD's are SATA.) MANY BIOS's have you set the Mode for all SATA ports the same, but some allow separate port Mode settings. The thing we are dealing with here is that XP (but not Vista) does NOT have any "built-in" driver for SATA or AHCI devices, and there are two ways to handle that. One, which is quite suitable for a storage drive only, is to ensure that you do load into Win XP Pro when it's running the driver your mobo needs for SATA (or AHCI) devices, and then it can handle them. The other work-around built into most BIOS's is to go into the BIOS Setup screens and ensure that the SATA Port Mode for the port(s) in question is (are) set to IDE Emulation, and NOT to SATA, AHCI, or RAID. Setting to IDE Emulation mode has the BIOS intervene and make the actual SATA HDD behave just like an older IDE unit that XP fully understands and can use properly.
Another thing to check: which SATA port is used for that "missing" unit? Many mobos have about 4 SATA ports, and many have an additional one or two that are provided by a separate controller chip. In the latter case, you usually have to ensure that you load into Windows the device driver for that chip and its extra ports. Since you have two HDD's and two optical drives already operating properly, is the last HDD on one of these "extra" ports that needs a driver installed?