Mosquitto broker - Installing on Master & Slave?

Hi,
I want to create a master - slave - slave system with with my 3 raspberry pi’s. On each raspberry I have an zigbee antenna.

Do I have to install the Mosquitto Broker on reach raspberry?
Do I have to install Zigbee2MQTT on each device?

Can I they run also seperatly?

I would recommend using one Zigbee2MQTT instance and rather use the others as routers if possible. If you have enough routers (bulbs and most mains powered zigbee devices works as routers) you shouldn’t have any issues if the area isn’t too big and complex.

if you want to run on the 3 machines, yes

I guess you only need 1 mosquitto, and your 3 Zigbee2MQTT’s talking to the same mosquitto. But that is dependant on what you exacly want to do.