Solution: Start with $a_1 = 1$. Compute $a_2 = M(a_1) = 1 - rac1^22 = rac12$. Next, compute $a_3 = M(a_2) = rac12 - rac{(rac12)^2}2 = rac12 - rac18 = rac38$. Thus, the value of $a_3$ is $oxed{\dfrac38}$. - Dyverse