Help with a pulley problem with 3 masses

[Delta2]

It seems each acceleration is being taken as positive in the likely direction of movement: 1 to the right, 2 to the left, 3 down.
For each unit of distance 1 advances, 3 drops 1 unit.
For each unit 2 advances, 3 drops 2 units.

Thanks @haruspex but I still don't understand
if "For each unit of distance 1 advances, 3 drops 1 unit" this means that $|\vec{a_1}|=|\vec{a_3}|$ right?
Also I don't understand how the following holds:
"For each unit 2 advances, 3 drops 2 units"

 

[haruspex]

Thanks @haruspex but I still don't understand
if "For each unit of distance 1 advances, 3 drops 1 unit" this means that $|\vec{a_1}|=|\vec{a_3}|$ right?

Only if 2 stays put. Similarly, 3 dropping 2 for each unit 2 moves is on the assumption that 1 stays put.
So in algebra it's $\frac{\partial a_3}{\partial a_1}=1$ etc.