Can a 3 phase solid state relay be used instead of 3 single phase relays?
In a 3-phase electromechanical relay, there are three contacts that are electrically isolated from each other but are operated simultaneously by a single electromechanical actuating mechanism. There is no means provided to open or close one contact without the other two assuming the same state simultaneously. There is also very little possibility for a failure that does not affect all three contacts in the same way.
The solid-state relay example linked in the question operated in much the same way. However it could possibly fail in such a way that one switch could fail to respond to an open or close command while the other two continue to operate normally. There may be other solid-state three-phase contactors that operated differently.