Generally speaking, I have to nock 1/8 - 1/4" higher for 3 under than for split fingers. My typical nocking point for split fingers would be 1/2", where my typical nocking point for 3 under would be 3/4". I would move up or down from there based on the results of bare shaft tuning.
I use tied-on nocks, which you can make out of floss, serving thread, or bowstring thread. Most of us who shoot 3 under nock both above and below the arrow, as arrows shot 3 under have been known to move.
When I measure the nocking point, I measure from the square position on the bowsquare (the line that would intersect the shelf or arrow rest) to the bottom of the nock that is above the arrow.