Since the Arma 3 launcher has been constantly improved over the last few years, we recommend using it.
We are aware that there are still players who use the A3Launcher http://a3launcher.com/. Due to the limited support, we strongly advise against using this launcher. Also, there are always errors caused by its use.
If you need help setting up or understanding the onboard Arma 3 launcher, please feel free to join the Arma 3 Discord: https://discord.gg/arma