What group of letters comes after PRU in the series DFI, HJM, LNQ, PRU?

Inside every group the second letter is two places past the first and the third is three places past the second: D 4, F 6, I 9; H 8, J 10, M 13; L 12, N 14, Q 17; P 16, R 18, U 21. The opening letters run 4, 8, 12, 16, going on by four, so the next opens at 20, which is T, and the group is T 20, V 22, Y 25. Letting the third letter be only two places past the second gives TVX; opening at S, place 19, gives SUX; letting the second letter be three places past the first gives TWY.