Activision released a DirectX 9.0 Fix, but 9.0 isn't out yet far as I know. Off the top o' my head, you can do a few things:
1) Reinstall dx 8.1 and see if that works.
2) Reinstall sfc3, again.
3) Forget about sfc3 not recognizing you have dx 8.1 and play the game anyways (does it even run btw? and if it does, how does the game perform?)
4) Or download the 9.0 Fix and see if that could actually affect your copy of sfc3 to recoginze you having 8.1.
I know this doesn't sound professional, but it's the best I can offer short of your asking for help from Activison or even Microsoft. Good luck with the problem.