I was just at JT with a buddy of mine who'd bought an Infinity Tactical 3-pt Sling, and, of course, he also needed help with it.
Attach it as you would a 2-pt, only a part of the sling is flat against the rifle, and the rest opens up into a D shape for you to put over your head. Now put your weak hand through the sling to hold the foregrip, with your strong arm outside it. Awesome. Now drop your gun. This should have the gun muzzle down and the stock and grip floating just over the middle of your chest. This does two things; easy to grab the gun once more and fire, and easy to reach your sidearm.
Picture 3 that Fish provided is the best example.
Just go to a JT with your gun and sling and they'll show you how to work it with a smile. This bunch of letters'll help you much. Since the sling you bought has many uses, you should see for yourself how it's used, and find out all its uses. Enjoy it. I know I'm getting one soon. (only ten bucks at your local JT)