You are bringing a new network online with three IS-IS routers using default Junos election...

A router’s priority for becoming the designated router is indicated by an arbitrary number from 0 through 127, which you configure on the IS-IS interface. The router with the highest priority becomes the designated router for the area (Level 1, Level 2, or both), also configured on the IS-IS interface. If routers in the network have the same priority, then the router with the highest MAC address is elected as the designated router. By default, routers have a priority value of 64.

https://www.juniper.net/documentation/us/en/software/junos/is-is/topics/concept/routing-protocol-is-is-security-designated-router-understanding.html