Got that on one of my bikes. Sram 3 x 7 (although it's a cassette not a freewheel). I think Sram call it a dual drive these days. Sturmey Archer do one too.
You can still buy the three speed cog for the SA hub gear.
They have and it's been done. It's a common touring mod on Bromptons, you fit a double chainset and move the chain manually, as the tubes are all in the wrong place for a front mech.A while back, a manny who ran a bike shop claimed that if you wanted extra gears with a rear hub, all you had to do was retain the double / triple chainring and the rear derailleur mech. His idea was that the rear mech would take up any chain slack during front changing and also act as the necessary tensioner required with a hub if your bike has vertical dropouts. He insisted it would work, but I'm not convinced and anyway, if it does, why has no-one else thought of it - ?
Yep you can do this. In fact a chain tensioner may be a cheaper solution than a rear derailleur to take up the slack.A while back, a manny who ran a bike shop claimed that if you wanted extra gears with a rear hub, all you had to do was retain the double / triple chainring and the rear derailleur mech. His idea was that the rear mech would take up any chain slack during front changing and also act as the necessary tensioner required with a hub if your bike has vertical dropouts. He insisted it would work, but I'm not convinced and anyway, if it does, why has no-one else thought of it - ?
Yup, had two bikes with DualDrive. A few recumbent manufacturers combine it with a triple crank for 81 gears and an even more humungous range. SRAM do a 10 speed version as well now.
Advantage being that as the bike in question is my all weather commuter, then most of the gears are safely tucked away from dirt, wet, winter salt etc.. Should be interesting to see if I get any odd comments on the setup - ! 
