Three equal weights of 3kg each are hanging on a string passing over a frictionless pulley as shown in figure. The tension in the string between masses II and III will be (Take g=10m/sec²):

Let us take the three masses and the strings as a system. Let the tension in the string be T. Equation of motion are as follows-
T - mg = ma
2mg - T = 2ma
⇒ a = g/3.
Now let us work from the ground reference for the block |||
Let tension in this string be T1
equation of its motion is-
mg - T1 = ma
⇒ T1 = 2mg/3
T1 = 20N