Orbits classifying extensions of prime power order groups

The strong isomorphism classes of extensions of finite groups are parametrized by orbits of a prescribed action on the second cohomology group. We study these orbits in the case of extensions of a finite abelian $p$-group by a cyclic factor of order $p$. As an application, we compute number and sizes of these orbits when the initial $p$-group is generated by at most $3$ elements.