@Turmanser 

This should work, but adding P9 which is older model may disable features present in Deco mesh consisting of X55s only. For example, "connection preference" feature will be unavailable if you add P9s to your Deco mesh. 

There is another option: get pair of powerline adapters compatible with Deco and one extra X55. Create Ethernet link with powerline adapters through the wall. Connect powerline adapter in Room 3 to new X55 and connect second powerline adapter to your home network, by Ethernet cable. 

With powerline adapters, the requirement is for them to be on same circuit breaker, other than that you have substantial flexibility where to place second powerline adapter in your house.

Not every powerline adapter is compatible with Deco backhaul. Choosing from TP-Link brand is recommended. Also, I recommend you get powerline adapters that have single Ethernet port. Adapters with multiple Ethernet ports on them have higher chance to be incompatible.

With number of Decos you have, you can test that it works, and speeds, by just buying powerline adapters set, with one of your Satellite X55 you already have moved to Room 3 for testing.

If it works to your satisfaction, then you can buy additional X55.