I'll bet that your feet were staggered (left foot forward if you're a righty). Stand with your feet in a line perpendicular (or nearly so) to the line of the shot. Have your left hand in the crook of the sticks and your left arm almost completely extended. Hold the rifle butt tightly against your shoulder. All of this should greatly solidify your hold.
Personally, I strongly prefer two sticks instead of three so that I can lean them back towards me.
RS
