There are several ways to do this but the easiest way is described here, there's also a harder way to do it but it's way too much of a hassle to go through and has a higher chance of not working at all.
Step1: Make sure that you have legitimate versions of both ArmA2 (preferably the Combined Operations version) and ArmA3 installed
Step2: Get sixupdater from
http://play.withsix.com/Step3: Get the AllinArma (AiA) mod for ArmA3 from the sixupdater mod list
Step4: Get ALL ACE mods (@ACE, @ACEX, @ACE_RU, @ACE_USnavy, @ACE_SM) and the @CBA mod for ArmA2 on sixupdater
Step5 (here's where it gets tricky): Select ArmA3 in sixupdater again and go to your mod list, create a collection named ACE and move the AiA mod from the local - mods folder to there, activate the collection and you should get a local - AllinArma folder
from there find your ACE mods and drag them to your ACE collection, in the end your sixupdater folders should look something like this
Step6: Done! Now launch the game *note: ACE mod adds a new startup menu which conflicts with the original ArmA3 menu, while you can see both of them and they both work it's easier to navigate using the original ArmA3 menu
you WILL get some errors when you launch a mission but you can ignore them and most things will still work correctly
