Why 3 equation is not balanced and only the upper two equations are balanced why is it so? It is inside the chapter and I want to know that why they had done this?

Dear Student,

As such it is expected that the chemical equation should be balanced. However, in this case I suppose they must have missed balancing it.

Here is the balanced equation:
Mg + 2HCl (dilute) --> MgCl2 + H2

Regards