As Zone 2 is a stereo output, I assume you are wanting to power 3 stereo pairs. Most people doing that use a multichannel zone amplifier like those available from Parasound, Rotel, Niles, Russound, etc. These amps have the ability to bus or link channels together to use the same stereo input.
Of course if you had 4 identical stereo speaker pairs, you could wire them in a series-parallel configuration to present a load equal to the impedance of a single speaker, and drive them all with a stereo amplifier (with of course no option to have level adjustments between pairs).
Some receivers can only output analog signals to their Zone outputs, so be sure to check the particular receiver to see if digital sources are also sent to Zone 2.