What group of letters follows ZWT, WTQ, TQN, QNK?

Within every group the letters fall three places at a time: Z 26, W 23, T 20; then W, T, Q; then T, Q, N; then Q 17, N 14, K 11. Each group also opens with the letter that stood second in the group before it, so the next group opens with N 14 and runs N, K 11, H 8 — that is NKH. Letting the last letter fall by only two gives NKI; letting the group open with M, three places past Q rather than at the second letter of QNK, gives MKH; letting the middle letter fall by only two gives NLH.