From one finger on each MTB lever it didn't take much to be comfortable with the following
One finger on the brake lever...one on the clutch lever. Can break traction on all my bikes with one finger braking and one finger clutching seems to work well. I tend to preload the shifter so it only takes a whiff of clutch to shift.
I like the security of three fingers on the grips, especially in dirt where an obstacle may disagree with you on direction of progress!
Wouldn't like the increased reaction time associated with no fingers on the brake.