@KlimczakM 

Any configuration can work in AP mode: all Deco connected to router, all Deco connected to switch, hybrid where some Deco connected to switch and some directly to router. I have hybrid configuration at my house: one Deco connected directly to router and other two - through switch.

Choosing between X20 and X50, with what you described, I would recommend Deco X50. For the following two reasons:

1. Deco X50 is newer model, and it may provide faster WiFi if mobile device supports it. 

2. Not every networking switch is compatible with Deco Ethernet backhaul, if you don't need switch with Deco X50 - that is preferred configuration.

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Having answered your question, I have mine. If I understand you correctly, you plan to connect all three Deco to router. Routers usually have four Ethernet ports, so why do you need a switch with three Deco X20? 

  @Alexandre. thank you for the answer. I want to connect 2 Decos' in AP mode to the 3rd Deco in router mode. There won't be more devices, just modem + 3x Deco.

KlimczakM wrote

  @Alexandre. thank you for the answer. I want to connect 2 Decos' in AP mode to the 3rd Deco in router mode. There won't be more devices, just modem + 3x Deco.

  @KlimczakM 

TP-Link has two modes for Deco mesh: Access Point or Router (default). I thought you were planning to run Deco mesh in Access Point mode, but you meant something different.

"Translating" what you said to TP-Link terminology: "I want to connect 2 Satellite Decos to the Main Deco, with Deco mesh running in router mode."

My recommendation is still the same and for the same reasons I mentioned: go with Deco X50s.