White Industries claims to be close to releasing a dual fixed cog. They also ellude to it being matched to a special hub. It would almost have to be.

I've asked some 3-speed hub manufacturers if they can modify a 3-speed to make it fixed.

They get a very funny look on their faces.

It took me eighteen months of talk to get them to understand flip/flop fixed/fixed hubs. So don't hold your breath for a 3-speed fixer.