Find the dissimilar term.

3X5, 4U3, 2R4, 3O2, 1M3.

The logic followed here is:- 3 × (sum of digits given on left and right side) = positional value of the alphabet given in the middle. 

1. 3X5 →  3 × (3 + 5) = 3 ×  8 = 24 = X(24)
2. 4U3 → 3 × (4 +3) = 3 ×  7 = 21 = U(21)
3. 2R4 → 3 × (2 + 4) = 3 × 6 = 18 = R(18)
4. 3O2 → 3 × (3 + 2) = 3 ×  5 = 15 = O(15)
5. 1M3 → 3 × (1 + 3) = 3 × 4 = 12 ≠ M(13) 

Here, 'IM3' is odd among the given options.

Hence, "Option - (3)" is the correct answer.