Stronglifts already incorporates 3x5. When you stall and deload twice on an exercise, you switch to 3x5. When you stall and deload twice on 3x5, you switch to 1x5. This may mean you're doing 3 reps of one exercise and 5 reps of another. Starting from a low weight you'll see pretty fast linear gains on 5x5. I say stick with 5x5 until you stall twice. Why do you want to change it?