CCommunity
The logic here is:
every set of letters follows the logic: x : x + 1 : x - 1 now (x - 1) - 1 forms the middle letter of next set.
TUS: T, T + 1, T - 1 : Next set: (T - 1) - 1 i.e., R will be the middle letter of next set.
QRP: Q, Q + 1, Q - 1 : Next set: (Q - 1) - 1 i.e., O will be the middle letter of next set
NOM: N, N + 1, N - 1 : Next set: (N - 1) - 1 i.e., L will be the middle letter of next set.
KLJ: K, K + 1, K - 1 : Next set (K - 1) - 1 i.e., I will be the middle letter of next set.
HIG:
HIG: H, H + 1, H - 1 : Next set (H - 1) - 1 i.e., F will be the middle letter of next set.