3. Let $A, B$ be two subsets of the set $\{1,2, \cdots, 20\}$, $A \cap B=\varnothing$, and if $n \in A$ then $2 n+2 \in B$. Denote $M(A)$ as the sum of the elements of $A$, then the maximum value of $M(A)$ is $\qquad$ .
$2 n+2 \leqslant 20 \Rightarrow n \leqslant 9$, which means we can only select from the array $(1,4),(2,6),(3,8),(4,10),(5,12),(6,14)$, $(7,16),(8,18),(9,20)$. Thus, taking $A=\{4,5,6,7,8,9\}$ satisfies the condition. Therefore, the maximum value of $M(A)$ is 39.