Legato playing Part 1

I can't get my head around playing 4 notes per string triplets or 3 notes per string 16ths as in exercises 3 and 4. No matter how slow i run the metronome my mind and picking hand defaults to 3 notes per string triplets or 4 notes per string 16ths and picking a new string on every beat. Especially when moving up the frets and down the strings e.g. the second half of exercise 3. Any advice other than 'go slowly' - which isn't helping !