My wife wants to do a bit of cardio to keep fit and lose a bit of weight, she has a bad back and thinks that borrowing my turbo will suit her over running (she used to be an awesome runner!)
Would something like a hub geared 3 speed work okay with TR?
If you can get it rigged up to a smart trainer and use erg mode for the TR workouts then sure!
You might find that difficult though, it might work on a wheel-on trainer if the frame spacing is right (i.e., the frame can be locked into position on the turbo), but if you’ve a wheel-off (direct drive) trainer it might be hard if not impossible, as they are usually designed around use of 9-12 speed hubs.
That new Wahoo Rollr smart roller thing where the front is locked in place and the rear wheel sits on rollers might work - probably the most sensible use case for it I have come across