That is pretty much my laptop specs. I could barely get ARMA 2 to run comfortably. ARMA 3 graphically is easier to run but it seems to eat up more resources when you try and play larger missions, especially on Altis.
The ARMA 3 copy I brought had the game on it. Save a shed load in download time. Still have to update it more then likely though once installed. I still have a 1.2GB update to get through before the weekend :/
FYI there's a free weekend for it on Steam starting tomorrow, you could set it to download over friday and be playing it by maybe Saturday afternoon depending on your internet connection