The effect of a third type of B-site cation on the temperature- and pressure-induced structural changes in ABO3-type relaxors has been studied by in-situ Raman scattering and X-ray diffraction on 0.72PbSc0.5Ta0.5O3—0.28PbSc0.5Nb0.5O3 (PSTN) and 0.78PbSc0.5Ta0.5O3—0.22PbSnO3 (PSTS). The incorporation of Nb into the PST matrix is an isovalent substitution for Ta5+ on the ferroelectrically active B-site, whereas the incorporation of Sn4+ is a coupled aliovalent substitution for both types of B-cations in the host system. Both PSTN and PSTS exhibit the characteristic temperature T*, which for PSTS is shifted to slightly lower temperatures. The incorporation of Nb and Sn into the PST matrix smears the transformation processes near and below T* and suppresses the ferroelectric long-range order at low temperatures. Both PSTN and PSTS exhibit a continuous pressure-induced phase transition. The high-pressure phases have the same structural features as for pure PST: suppressed B-cation off-centre shifts, enhancement of coupled Pb—O ferroic species, and long-range order of anti-phase tilts of the BO6 octahedra. Nb-doping shifts the critical pressure pc from 1.9 GPa for pure PST to 2.5 GPa, whereas Sn-doping lowers pc to 1.3 GPa. The incorporation of both Nb and Sn decreases the degree of the overall pressure-induced structural distortion, with the effect of Sn being more pronounced than Nb. The dilution of the B-site cation system with Sn4+ results in local electric and elastic fields which reduce the coherence between nanoregions with anti-phase octahedral tilting and shifts the pressure at which long-range order of the tilts occurs to ∼4 GPa.