A3 is an unofficial, easy to use launcher for Arma 3 that automatically downloads mods when joining any modded server and helps you easily manage all your mods.

A3 Launcher can be downloaded here:
https://a3launcher.com/

Step 1: Press the green "Download Now" button on the website.

Step 2: Open the download file (there will be a User Account Control pop-up, simply hit yes).

Step 3: Choose which folder you would like A3 installed into.

Step 4: Leave this as is and hit next.

Step 5: Choose whether or not you want a desktop shortcut (recommended you have one).

Step 6: Installer should take seconds to complete.

The launcher itself is pretty straightforward. You can find the desired modded server, join it and the launcher will automatically download the required mods. Once finished, you will be able to start playing in any modded server of your choice. Have fun!