Calculating Tension in a Pulley System with Three Masses

[Anza Power]

We have the following system in ideal condition (pulleys and rope are mass-less and frictionless)

We need to find the conditions in which M₃ will go downwards...

Ok so here's what I did:

M₃g > T

And this is what I'm not sure of:
T=(M₁g + 0.5M₂g + M₃g)/3

So the answer which I came to was:

2M₃ > M₁ + 0.5M₂

But I'm a bit unsure, did I go wrong? how do I approach this question and how do I calculate the tension in the rope?

 

[Anza Power]

I am sorry to bump this but can someone just tell me if the tension calculation is right? I just need to know how to find the tension and I can solve everything (there are other parts in this question)