Question about Synology MR2200ac

This is possible, but you lose the main function of these APs: you won't have mesh WiFi. You'd be better off buying 3 other cheaper routers. By the way, can you really connect 3 routers directly to your modem (WAN)? With the mesh wifi, you only have 1 SSID, you don't have to change wifi when you move around the house.
To get a Mesh-Wifi, 2 of these APs have to be connected to the first one (unless you already have a router, e.g. the RT2600ac). They are then managed directly and exclusively by this router (the first one takes over the router function) (e.g. for system updates). The configuration is really simple. The router is connected to the WAN and the 2 other APs are connected to the router, either via cable (LAN, but then you need a switch, the MR2200ac has only one Ethernet port left) or wireless (WLAN).
You will hardly notice a speed reduction, unless you have a very high demand for data exchange. If this is the case, then you definitely need other faster devices, but these will cost a bit more than the Synology APs.
I hope it helps you.

These devices can also be connected to each other via Ethernet. You may need a small switch (e.g. Netgear with metal housing) for this. I have connected them this way on my RT2600ac. It is not clearly described anywhere in the user manual, but it is possible.
1) configure the devices as AP via WLAN (very easy, almost automatic)
2) Bring the devices to their place of use and test Mesh-Wifi.
3) After successful testing, connect the 2 APs via Ethernet and adjust the configuration on the router accordingly. The router recognises the location.
4) Enjoy your Mesh-Wifi.

You can still save yourself the short interruption by configuring these APs as mesh Wifi. You will almost never notice a restriction in the bandwidth. These devices are fast enough for that.
2nd advantage, you will only have to manage 1 device and not 3. The 2 APs automatically take over the settings from 1. You can have system updates done automatically at night.