Theoretically it shouldn't be that hard, however there being so many different mfrs of HDDs, it would be kind if hard to say that making all of them work wouldnt' be major undertaking. Yes I agree that for the most part, the underlying platforms for HDDs and USB should be mainstream standards used between all HDDs and the USB connections. However when it comes to Shareport and these routers. I seen various problems in using this function and have personally chosen to avoid use it unless I'm helping to test a device like I have here in the forums. Most of the time it works and does for my 2Tb My Book. I have seen it were FW was involved and breaks Shareport like we are currently seeing on the DIR-655 v2.07 FW. It's possible there could be an issue with FW and SP and your particular model HDD drive. If Dlink said only 500Gb drive is supported, I presume this means physical drive space on one drive. I'll have to go look at my drive however I think it's got 2 500Gb drives and uses the SW to gain the 2Tb size in some RAID configuration. I wonder if your 3Tb drive has larger physical drives bigger than 500gb. Possible?
One other suggestion, do you have a smaller USB drive that you can test out? Friend or get one at a store with a refund policy. If you can find an actual 250 or 500Gb physical sized drive to test out. If this works on your router, then you'll know.