It is possible, but IMO not a good idea. The tranny was beefed up several times (even before increased torque of the 6 cylinder engine).
Not certain, but you may have to cut down the input shaft just a bit. It got shorter, although I'm not sure why offhand. But ISTR the pilot bearing moved closer to the gearbox at some point, so it may be the long shaft would bottom against something. A TR6 box will rest flat on it's face on the floor, while a TR3 box won't.
The front cover of the gearbox (where the throw-out bearing carrier rides) also has to be cut down for use with the diaphragm clutch. The smooth portion should be only 2" long, not 2-3/8" as the TR3 was.
If it's from an earlier car than TS50000, then you'll also want to do something about filling the gearbox in the car. TR2-early 3A had a dipstick in the top cover and a hole in the bodywork for access; but lacked the fill hole in the side of the gearbox. I'm not certain, but it may be that the center console blocks where the bodywork hole would be on a TR6.
I assume this is non-OD. There may be other issues with installing an A-type OD in a late TR6. The OD itself was modified for use with IRS as well.