1. The problem is to evaluate the expression $3 - 1_2$. 2. The notation $1_2$ means the number 1 in base 2 (binary). 3. In base 2, $1_2$ equals 1 in decimal. 4. So the expression becomes $3 - 1$. 5. Subtracting, $3 - 1 = 2$. 6. Therefore, the final answer is $2$.