Its a Vertex mobile being used as a base station
OK, no big deal. We did this in our fire station with a Motorola CDM-1250.
We took an audio feed off the back of the radio. That was fed into one of the RDL labs mixer (link above) to allow two CDM-1250's to feed the system, one for our dispatch, one for an adjoining agency. The amplifier fed out through the station. There were volume controls and speakers in the various rooms.
Pretty simple and easy setup. If you have just the one radio, then you don't need the mixer. For 3 speakers, you don't need a big amplifier.
Heck, you could get an old home stereo tuner/amplifier that had two sets of speaker outputs. That would give you the ability to run 4 speakers. Use some simple 8Ω speakers. You could probably find a tuner/amp at Goodwill pretty cheap.