Potential Energy is energy stored in the system and ready (potentially) to be changed into, say, Kinetic Energy through movement.
The general expression for Gravitational Potential Energy is:
U
=
m
g
h
where:
m
g
is the weight;
h
is the height relative to a certain reference level (the ground in this case) choosen to have zero Potential Energy.
With your data we get:
U
=
3
⋅
30
=
90
J
