cingrisano, what sight picture are you using? Unlike many other gunmakers, SIGs are set up to use sight image #3 (see Mystro's post) so using either of the other two will result in shooting low.
Do you have night sights? Are there no other numbers on your sights? I don't believe MH3 is the sight #. As has been mentioned previously, 9mm SIGs normally have #8 sights front and rear. An increase of 1 number in the rear sight raises the POI by 2" at 25 yds and an increase of 1 number in the front sight raises the POI by 1".
Do you have night sights? Are there no other numbers on your sights? I don't believe MH3 is the sight #. As has been mentioned previously, 9mm SIGs normally have #8 sights front and rear. An increase of 1 number in the rear sight raises the POI by 2" at 25 yds and an increase of 1 number in the front sight raises the POI by 1".