3 bolt are the original imperial threaded ones fitted to RRC, I'm told they are stronger.
4 bolt are the later metric ones fitted to all Discos
6 bolt are found on some Defenders, IIRC they are H/D, never seen one.
The only differnece worth worrying about is the later ones had a 6mmm hole at the back of the output shaft that lines up with a slot in the drop arm, to find the centre fo travel. Otherwise wind it all the wat to the left (drag link removed) then wind it 2 full turns to the right, that's the mid-point.