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
