From what I have found I’d recommend trying reinstalling the game from scratch, some say that it can help but sometimes it doesn’t. Another issue some seem to reference is
this if you have multiple people at your house using Steam to play games. Another site that has a lot of resources can be found
here. The ones that stand out to me that may be causing you issues would be the firewall / anti-virus hampering the game, or routers / firewalls identify the Steam multiplayer server queries as spam traffic. If you haven’t already I would recommend
port forwarding Arma, it’s something that I’ve been doing with pretty much all of my multiplayer games for years since it can help alleviate issues. I’d also recommend exempting Arma 3 or even Steam from the firewall (
info) so that it will just leave it alone. I would also recommend exempting your entire Steam folder from your anti-virus scans since I have had issues with my anti-virus, Norton, deleting files from it before, specifically Arma 3 files that were required for the game to even function.This issue is definitely something on your end with the network or local system and these are good steps to take to try and stop it from happening again.
As a side note, are you using a WiFi dongle for your connection? And if you are is there perhaps a microwave being used nearby? (Yes this is a real issue that used to cause me to disconnect all the time because the microwave and WiFi dongle operated on the same frequency.) Also what is your internet speed that you have?