Answer:
The tension on each end string = n
a = F / m = 0 given zero m = (3 + 4) kg N for no acceleration of the combination
The force on the middle string is also n because no acceleration is involved
For any one block to be accelerated there must be a net force on that block, in which case the force exerted by the string (in the middle) is not the same as the force exerted by the end strings
F = M a applies to any one block or combination of blocks
T the net tension on the middle string is the same on both ends because a net force on a mass of zero would produce infinite acceleration