This basic design concept remains, for the most part, unchanged today. However, even though the function of single-phase and three phase relays are quite similar, their applications can vary significantly. This is primarily due to the characteristics of three phase power circuits and the attributes/ demands of three phase loads, particularly inductive loads.
Three Phase Solid State Relays & Their Application in Three Phase Motor Circuits
Three phase Solid State Relays (SSRs) have been available since shortly after the introduction of Single Phase SSRs in 1970. The first of these relays were nothing more than 3 individual SSRs with a common input connection packaged in a single housing.