Hi,
Andy, it depends on hardware scheme. For example, you connect TX pin of MCU1 (here: master) with RX pin of MCU2 and MCU3 and so TX pins of MCU2/3 to RX pin of MCU1. Now explain me how it may work if MCU2 comes to be as master and MCU3 still keeps as slave? They both have TX connected together and transmission from MCU2 comes to TX of MCU3 (= You should then make some additional hardware to communicate all their TX/RX signals.
Even if master is always fixed so anyway for many nodes it is need to use extra-hardware as buffer because output current of a chip is limited.

There is another way - connect all the MCUs via Ring scheme: TX/MCU1->RX/MCU2/TX->RX/MCU3/TX/->RX/MCU1. The idea is simple - each node retransmits passthrough messages of another node and rejects own ones after they come back via Ring. Yes, it takes additional time (but easy may be done inside UART ISR) and as well there is another problem - if any node fails/switched off then communication will be terminated at all.
Good days!