  In this paper we study the reducibility, composition series and unitarity of
the components of some degenerate principal series representations of
$\RMO(p,q)$, $\RMU(p,q)$ and $\SP(p,q)$. This is done by realizing these
representations in paces of homogeneous functions on light cones and writing
down the explicit actions of the universal enveloping algebra of the group
concerned.
