There are two things that cause nock high. The first is where your nock point is too high. This isn’t ideal, but not really a problem, either, as a true nock high is very consistent and repeatable, and won’t affect your accuracy (much). The second is where the arrow shaft bounces off the shelf, resulting in what we call a false nock high. A false nock high is detrimental to your accuracy.
A false nock high can either be caused by a too low nock point or form issues. Assuming you already have a reasonably high nock point, like 5/8”, it is better to work on form issues first, because it is hard to determine where your nock point should be if you’re not shooting correctly.
Many people put too much string pressure on their ring fingers. Most of the string pressure when you draw the bow should be on your middle finger, next your index finger, and very little on your ring finger. Learn to come all the way to full draw with back tension. Shortdrawing the bow and overusing your arm muscles is a common error that makes it hard to get a clean release.
A problem that only occurs with the split finger grip is holding your string arm elbow too high, which causes you to press down on the nock with your index finger as you draw the bow. This can actually result in a bowed arrow that springs back when shot, which in turn results in another form of false nock high.
Too much pressure on the ring finger is a worse problem for a 3 under shooter than a split finger shooter, because the ring finger of a 3 under shooter is further away from the nock than it is for a split finger shooter, giving it more leverage to do bad things to your shot.
All of the things I discussed will result in nock high shots if they are not corrected.
Once you feel like you have a good consistent string grip and draw, you can start experimenting with nock point. Most people like to start with a nock point high enough that there is no possibility of getting a false nock high, like 1”. A 1” high nock point should result in a natural nock high. Then work the nock down the string maybe 1/16” at a time, hopefully noticing with each movement that the nock high is getting less and less. Continue to do this until all but a tiny bit of nock high has been eliminated, or else you have reached a point where the nock high starts to increase again, meaning it has become a false nock high. Either put your nock point where there is only a tiny bit of nock high, or at the lowest point you can achieve before it switches to false nock high.