Three of the following letter-clusters are alike in some manner and hence form a group. Which letter-cluster does not belong to that group?

HDX

1   2    3    4   5    6   7     8    9  10  11  12  13

 A   B   C    D   E    F   G    H    I    J    K    L   M

 ↓    ↓    ↓    ↓    ↓     ↓   ↓    ↓     ↓   ↓    ↓    ↓    ↓

 Z   Y    X   W   V    U   T    S    R   Q   P    O   N

26  25  24  23  22  21  20  19  18  17  16  15  14

THE LOGIC FOLLOWED HERE IS;

(1st and 3rd letters = Opposite letters)

(2nd and 3rd letters = Difference(+3) between the positional value)

1) UCF,

U(21) → F(6)

C →(+3) →  F

2) ZXA,

Z(26) → A(1)

X →(+3) →  A

3) NJM,

N(14) → M(13)

J →(+3) → M

4) HDX,

H(8) → X(24)

D →(+20) → X

Thus, HDX does not belong to that group