For example:
The x+y*z is interpreted as
(x+y)*z

by this grammar (if we use leftmost derivations) and as

x+(y*z)

by G1 or G2.

That is, both G1 and G2 grammar handle the operator precedence correctly (since * has higher precedence than +), while the G3 grammar does not.

In next post, I will explain the most interesting part of context free grammars that is ambiguous grammar.