Home LAN

I have 3 Tp-Link routers in the LAN. Each has a unique IP. Example= ??? . ??? . 40 . 1 through ??? . ??? . 40 . 3

My issue is that I can access the internet from each router, however, I cannot ping the 40 . 3 router from the other two.

All routers are cabled together with 40 . 1 being the primary.

Any thoughts?