Personally, I would think 3 volumes is going to give you a lot of foaming and problems when bottling.
I have been in the same situation you are in. What I would recommend is to let the beer flatten out as much as possible, like you said. I then make a priming solution on the very low end of my desired range (realizing that there could be CO2 in solution as well as a couple extra points from brett). So, if the range I am shooting for is 2.8-3.2 volumes.... I will prime based on the 2.8 with the knowledge that the low end will be just fine, and even if I get a bit more than I thought, it still is not going to make bombs or be way over carbed.
I mix the priming solution and put it in a purged keg. I jump the beer from the current keg down the dip tube of the empty keg with priming solution. This helps to mix it up pretty well in my experience. Then I use a beer gun with CO2 pressure to fill bottles and cap.