Solution :
$${z_2} = \frac{{2{z_1} + {z_3}}}{{2 + 1}}$$
$$ \Rightarrow {z_2}$$ divides the line segment joining $${z_1},{z_3}$$ in the ratio $$2 : 1$$ internally.
Note : If $${z_1},{z_2},{z_3}$$ are related by $$a{z_1} + b{z_2} + c{z_3} = 0,$$ where $$a + b \pm c = 0,$$ then $${z_1},{z_2},{z_3}$$ will be collinear points.
ALTERNATE SOLUTION
$$\eqalign{
& {\text{Given, }}2{z_1} + {z_3} = 3{z_2} \cr
& {z_2} = \frac{{2{z_1} + {z_3}}}{{2 + 1}} \cr} $$
So, $${z_2}$$ divided the line joining the point $${z_1},\,{z_3}$$ are in the ratio $$1 : 2$$
So, $${z_1},{z_2},{z_3}$$ are collinear.