1. uninstall all ArmA 2 tools (be careful some of these are named stupid like “Sound Tools uninstall”)
2. open steam client
3. choose library → tools → arma 3 tools
4. use RMB → Install game…
5. choose your steam library if you got one (might be good idea to install tools on separate location)
6. agree to eula
7. use RMB arma 3 tools → Play game…
8. accept license with yes button
9. close arma 3 tools program (not steam client)

All done!

Never launch arma3 tools from Arma3Tools.exe launcher, always use short cut to desktop or from batch file. You need to run one of the tools to activate registry entries, for example start Addon Builder once.

To complete your installation see Setup P: Drive.

Never ever run Steam client → ArmA 3 Tools → Prepare virtual P: Drive, it will delete all your files in the P: drive if it already exists, this is very dangerous command, please be careful not to ever run it.