How to organize a local network of 3 routers with 3 modems on them?

Question

Yota

It is necessary to organize a network and Internet access in three rooms. There are three zyxel keenetic routers and three yota modems. How to configure these routers so that the routers distribute the Internet to three cabinets from yota modems, while having a common local network and access to file storage in the third room? 3 modems to distribute the load, and support the Internet speed at a normal level on 23 computers.

Answer 1

We shove all computers into one subnet, on computers in each building we register their router as a gateway.
In general, it’s a little unclear how you are going to combine all this into physical. level, switches, etc.
With such questions, it is considered cultural to draw a diagram.

Answer 2

As you were told, put everyone on the same network, turn off DHCP everywhere (you will have to assign it manually), and assign static IPs and gateways depending on the network ownership.