I've not done it myself, but from what I've heard it is a real bear to get out. For whatever reason, the plug is thicker and has much taller sides than the usual 'freeze' plug. The TR3 also has very limited access in that area.
My approach would probably be to punch a hole in the center, then use a suitable 1/2" drive socket and 1/2" diameter lag bolt as a puller. If that won't move it, you may have to cut it to pieces.
Afterwards, wash the hole with carb cleaner, then change the oil and install a magnetic drain plug. That should remove any swarf that fell inside, or at least trap it on the plug.