You normally need around 3,3-3,4V at the emitter in order to give it around 3-3,5A.
You want to be quite exact, because 0,1V too high or too low will give you output well below or above what you are seeking. Best way is to measure amps at the emitter in the setup you are using, then you can check battery voltage once you have reached the desired amps.
Easiest way to direct drive an XM-L emitter at 3-3,5A, is to go with an XM-L2 on copper and green protected NCR18650B fully charged. Peak output is usually around 3,5A (if you do not have much resistance in springs and button), output will drop quite soon.
What is the point of this test? And if its just a test and not for use like you say, why are you asking others for the answer to the test? Just test it yourself in your own test setup. That is the best way to get the correct answer.
You are right, I should just test it myself, except I don’t want to fry this XML, while getting a measurement. I guess the best way I should do it, is to start at a low enough voltage to get a safe reading, and then keep charging the battery a little, and keep on measuring the emitter draw, and keep charging the battery until I reach my 3A emitter draw.
Because of the variables, the ONLY way to get 3.0A is with a 3A current source (driver). That is why these lights use current regulators, not voltage regulators. I have reaffirmed this myself in one of my own builds.