You are bringing a new network online with three MX Series devices enabled for STP.

https://supportportal.juniper.net/s/article/EX-Identify-the-Root-Bridge-in-a-Spanning-Tree-STP-network?language=en_US The root bridge in a spanning-tree network is the bridge with the smallest or the lowest bridge ID.

In the absence of a manually configured priority, the Spanning Tree Protocol (STP) elects the root bridge based on the lowest bridge ID, which is a combination of the priority and the MAC address. The device with the lowest MAC address will have the lowest bridge ID and thus be elected as the root bridge.

References

Juniper Networks Technical Documentation on STP