Embodiments of an isolator are provided, as are embodiments of a spacecraft isolation system employing a number of three parameter isolators. In one embodiment, the isolator includes a damper assembly and a thermal compensator external to the damper assembly. The damper assembly includes, in turn, a damper housing and a first hydraulic chamber, which is located within the damper housing and which is configured to contain a damping fluid. The thermal compensator includes a thermal compensator chamber, which is fluidly coupled to the first hydraulic chamber and which is configured to exchange damping fluid therewith during operation of the isolator. A thermal compensator bellows bounds an inner circumference of the thermal compensator chamber such that the bellows is externally pressurized when the first hydraulic chamber and the thermal compensator chamber are filled with the damping fluid.