2D magnetism in triangular lattices has already shown a potential for hosting exotic magnetic states. Control of these magnetic states, both in terms of magnetic properties and in terms of charge doping would be the next step. This makes materials which combine triangular lattice magnetic layers with layers hosting interesting structural or electronic properties particularly useful. PrCd${}_3$P${}_3$, studied in this work, is one of a family of materials where triangular lattice layers of magnetic rare earth ions alternate with semiconducting hexagonal CdP layers. Using Raman scattering spectroscopy we uncover a structural instability in the CdP layers, associated with a soft mode behavior of a phonon in these layers. Raman scattering detects crystal electric field excitations, and confirms a singlet ground state for Pr${}^{3+}$ and splitting of the doublet levels as a result of the structural instability in CdP layers. While Pr${}^{3+}$ is non-magnetic in PrCd3P3 we speculate that this family of materials can realize control of the magnetic rare earth layer via control of the coupled non-magnetic CdP layer. Additionally, we expect that these materials could become ferroelectric or multiferroic under strain that would relieve frustration.