Virtual Programming has released a new version of their ARMA 3 beta which brings the game in sync with the Windows version for the first time. The new beta also provides support for the Laws of War DLC.
As stated by Bohemia Interactive themselves, this means the game is at least temporarily compatible with the Windows version when it comes to multiplayer. However, there are no guarantees that the Linux port will remain in sync with the Windows version when the game next updates. This also apparently does not lift the beta status of the ports, meaning they are still considered experimental.
If you already purchased the ARMA 3 port then this should be fairly good news to you, since you will now have a chance at playing on the more mainstream multiplayer servers. But, like I pointed out, there are no guarantees and considering how long it has taken for the port to catch up to the Windows version I don't think this will be something that will convince you to buy the game if you hadn't already. Time will tell whether or not the Linux port will remain adequately in sync with the Windows releases for multiplayer to remain compatible.
If you want to check out ARMA 3 you can find it on Steam.