(1) According to the fact that the average speed is equal to the instantaneous speed, there are:

The speed of block b just after passing through the ring is v = dt.

(2) According to the meaning of the question, the gravitational potential energy reduced by the system ABC is converted into the kinetic energy increased by the system, namely:

mgh+Mgh-Mgh= 12(2M+m)v2，

Namely: mgh= 12(2M+m)d2t2,

(3) According to mgh= 12(2M+m)d2t2, the formula shows that besides the ruler, photoelectric gate and digital timer, a balance is needed to measure the quality of metal sheet.

So the answer is: (1)dt? (2)mgh= 12(2M+m)d2t2，

(3) Balance