We start by looking at Block 1
is the force exerted by block two on block one and it is equal to the force exerted by block 1 on block 2.
Now for Block 2:
Lastly Block 3
We now have 3 equations, one from each block, and three unknowns ( 
).
We write them out and add them up
Now we have to simplify what we have to the form in the question:
