Alright, please specify what you mean by "tag", and "arma 3 profile/squad selections". From what I can get through this here is my advice.
You need to go to here!
Once you have uploaded and created your unit you can select by either going to your "Arma 3 Launcher" and then hovering over your chosen unit on the menu, then in the top right select the box and left click it. This will select your unit, alternatively you can select to not use a unit (Units by the way only show a patch on your left arm and a little tag at the end of your name in the chat) or use the same unit as last time, so you do not need to specify. Automatically it will select the oldest unit you've been in.
Now if you are using Arma Squads it will do no different from Arma 3 units, so in truth don't really bother. Instead us Units.
I tried to help, Towelie