@B3cks 

For your network, I would recommend all four M9 Plus (you can buy them in two 2-packs), instead of M9 Plus (3-pack) + 1 M3W.

Consistent networking hardware is always better. Every time TP-Link releases firmware upgrade for M9 Plus, all your Deco mesh units will receive bug fixes and new features. With M3W in the mix, you may have to wait till M3W firmware upgraded, too, to get new features enabled in your mesh. Which (M3W new firmware) may, or may not happen.

So, M9 Plus 4-pack strongly recommended.

Having said that, answering your questions.

1. Yes, you can arrange network that way.

2. No, presently you can not force slave Deco M9 Plus to connect to specific other Deco. That functionality comes in firmware 1.6, Deco M9 Plus is on 1.5. Note that it is not clear for us on this forum, when TP-Link will release firmware 1.6 for M9 Plus. 

TP-Link offers beta firmware for M9 Plus with what they call "improved source signal selection," to find about it follow link at the end of this post. With that firmware, you still can't configure slave Deco where to connect to, but presumably it'll be choosing Deco with strongest signal instead of preferring main Deco. That might work for your layout.

More information on that topic can be found here: [Deco] Manual Signal Source Selection

@Alexandre. 
 

Thank you Alexandre for your time and your clarifications.

I will take your advice and I'll buy 4 M9 Plus.

Would suggest me to setup in Router mode or AP?

@B3cks 

I would recommend to setup Deco in Access Point mode. This is how I run my Deco mesh: two M9 Plus and two M5.

In Access Point mode the router that you now have will keep managing home network, Deco mesh will provide WiFi coverage. This mode gives best performance and least headaches. You can connect anything anywhere to your wired home network or to free Ethernet ports of Main and Satellite Decos, and everything will work. If you have to reposition Satellite Deco later or find way to have some of them wired to home network, you won't need to make any changes to Deco mesh setup and layout.

Also, you can keep using Ethernet ports of your current router if there is something connected to them currently. For example, in my household, I have file and media server attached to one of Ethernet ports of ISP Router, no changes were required for devices to communicate with it when I deployed Deco mesh.

By the way, if you have TV cable(s) in your house, you may be able to use MoCA adapters to send Ethernet traffic through them, and connect some or all Satellite Deco units to these adapters - that should improve their performance.

Also, if you can have Ethernet through TV cable coming from Room 1, where it'll be connected to current router, to Room 3 - you can place Main Deco in Room 3, then Room 1 and Room 5 Satellite Deco will definitely connect to Main Deco in Room 3. For these three it'll be best possible setup.