Hi.
I would need to connect internet to 3 buildings from the main building wireless.
The buildings are about 50m away from the main building.
Which MikroTik devices would be suitable to connect them?
All buildings are in direct sight.
Because of the distance, having line of sight, you can use practically any device (120° minimum aperture). But you have to take into account how much bandwidth you want delivered through those links, just like if you connect directly to clients or a general station in each building
Definitely go for the wireless wire devices (60Ghz/11ad). They give you 1Gbps + throughput!
Do not use the Wifi devices for this if you have line of sight.
You can do one AP and 3 “stations” (or if you want max perf. you put
3 dedicated 1Gbps links between the building 1 and each other building.
For this application Mikrotik has the best product! You are lucky 
https://mikrotik.com/products/group/wireless-systems?filter&s=c&w=[%22w_ad%22]#!
wAP 60Gx3 AP : One 11ad AP that can handle up to 8 11ad clients → here total limit for all 3 buildings together is 1Gbps
or
“wireless wire”: for each building to building 1 connection separately (you need 3 of these packs) this gives 1Gpbs per building
For this distance (50meter) you do not need the “wireless wire dish”.
Hope this helps.