Find the dissimilar term from the following group:
3X5, 4U3, 2R4, 3O2, 1M3

Let's examine the relationship between the digits and the positional value of the middle letter ($A=1,B=2,\dots ,Z=26$):
* 3X5: Sum of digits = $3+5=8$. Multiplying by 3 gives $8\times 3=24$, which is the positional value of X.
* 4U3: Sum of digits = $4+3=7$. Multiplying by 3 gives $7\times 3=21$, which is the positional value of U.
* 2R4: Sum of digits = $2+4=6$. Multiplying by 3 gives $6\times 3=18$, which is the positional value of R.
* 3O2: Sum of digits = $3+2=5$. Multiplying by 3 gives $5\times 3=15$, which is the positional value of O.
* 1M3: Sum of digits = $1+3=4$. Multiplying by 3 gives $4\times 3=12$, which corresponds to L. However, the given letter is M (positional value 13).

Therefore, 1M3 is the dissimilar term.