When I went Brett-only with a vial of Brett Brux Trois, I had to step it up a couple of times to get a decent pitch. After that it fermented like a normal Sach, with only a slightly longer lag time of a few extra hrs. When Brett is used by itself, it doesn't throw the funk that you get when it's stressed as a secondary pitch.
With a 3L starter, there should have been quite the slurry of yeast to pitch. Did you see a lot, or was it more like the small amount from the initial vial?