I have recently retuned about 6 bows for 3 under. The average would be a nock point 1/8 - 1/4" higher than for split fingers. Since it is thought to be a benefit to have two nocking points when shooting 3 under, one idea to get in the ballpark would be to nock above your old nock (assuming you nocked below the nock for split fingers), and then set a new upper nock slightly above the arrow.
The biggest problem I've had is getting false nock highs when bare shafting. The bare shaft is bouncing off the shelf. When I raise the nock point another 1/8" (which sometimes seems excessively high), I begin to get a nock low and then a little higher and I get a true nock high.
My range of nock points (nocking above the arrow) for 3 under is 5/8" - 7/8"+ (I run out of marks on my bow square). I was afraid that such a high nock point would adversely affect the performance of the bow, but all my bows seem to work just fine when I get the nock in the right place.