WGT uses your best 25 ranked rounds (I believe) at the Master tier to determine your average. In order for your average to go down, you only need to shoot better than the worst of your best 25.
I'd assume that the 3 under you refer to is for 9 holes, not 18, which would equate to 6 under for 18, or at worst, a 66. Shooting a 66 with a 65 average isn't going to make your average drop much, if at all.