I'm trying to practice with and without a sling, I've invested in one of those single point slings to help me in the standing position to improve my accuracy, but I don't think I can count on having it all the time.
Three gun competitions are what I'm looking to get into as well.
I am not familiar with the requirements of three gun competitions involving a rifle. It may well be that a free floated barrel is not as important as it is in HP Service Rifle matches.
As to offhand shooting and slings. In HP matches, the sling is not used while shooting offhand....notice the shooter in the earlier picture.