If it has to be a diaphragm clutch, then you will need to get the flywheel either drilled & tapped; or replace with a later (TR4A) flywheel. Also change the TOB and its carrier, plus shorten the front cover a bit (about 3/8" IIRC) so the fingers don't hit it as they flex.
But I think there is a friction plate available that will fit the smaller spline and also the stock TR3 pressure plate. Unfortunately, I don't know the details. You might check with Canley Classics in the UK if someone else doesn't chime in with more info.