The 3:1 rule applies to Conventional Warfare and conventional thinking of tactics/strategy.
Down through the ages, as methods and technology altered the application of force during combat, perhaps that old rule has been outdated for the types of conflict we find ourselves confronting now.
Certainly in an Insurgency type conflict.....it is far more than 3:1 advantage the defending forces will have to have in order to prevail as it will have to secure and hold large areas with numerous defensive positions which are all vulnerable to attack as well as the lines of communication and supply.
That is the key to Asymetrical Warfare.....the advantage generally lies with the Insurgents to be able to initiate the action when it best favors them and puts the defenders at the disadvantage.
An excellent example has been given.....one guy with a rifle or LMG takes a pop at a helicopter, patrol, or convoy....then legs it back into a village or town...or does so from within the village surrounded by innocent civilians.....and now look at the amount of people, equipment, and logistics required to find him and neutralize that threat.
I think it is time to pack our stuff, fold our tents, and bring the Troops home. Once again....the Afghans have shown the outsiders that winning in Afghanistan can only be done if one does not overstay your welcome.