Three blocks with masses $m,2m$ and $3m$ are connected by strings, as shown in the figure. After an upward force $F$ is applied on block $m$, the masses move upward at constant speed $v$. What is the net force on the block of mass $2m$? ($g$ is the acceleration due to gravity)

According to Newton's Second Law of Motion:
$$F_{net}=ma$$
The problem states that the blocks are moving at a constant speed $v$. Constant speed implies that the acceleration $a$ is zero.
$$a=\frac{dv}{dt}=0$$
Since $a=0$ for all blocks in the system, the net force acting on each individual block, including the block of mass $2m$, must be zero.
$$F_{net}=2m\times 0=0$$