I shoot 3 under. Is it better to nock the arrow below or above the string nock..? I nock under so to push the the arrow against the brass thingy.
Yes, under is correct
These days, it is generally recommended to have nocks above and below the arrow when shooting 3 under, because the arrow can slide down the string on release, causing a high miss. Leave enough room between the nocks so the arrow can slide up and down maybe 1/32" , so it doesn't get pinched at full draw.
As mentioned, use one nock above if you decide to go with just one nock.
As McDave said, add a nock under the arrow. Slow motion video has shown that with 3 under, after release, the nock slips down the string trying to center itself. >>>------> Ken