A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature ($T_{rm c}$). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving nonmagnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe overlap with each other, the effects of two orders on superconductivity remain perplexing. Here we construct the three-dimensional electronic phase diagram, temperature ($T$) against pressure ($P$) and isovalent S-substitution ($x$), for FeSe$_{1-x}$S$_{x}$, in which we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended nonmagnetic tetragonal phase emerges, where we find a striking enhancement of $T_{rm c}$. The completed phase diagram uncovers two superconducting domes with similarly high $T_{rm c}$ on both ends of the dome-shaped antiferromagnetic phase. The $T_{rm c}(P,x)$ variation implies that nematic fluctuations unless accompanying magnetism are not relevant for high-$T_{rm c}$ superconductivity in this system.
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