PaulHoule is correct; potassium and iron are dangerous in excess. Magnesium you can just take in a separate supplement since you really want more of it than easily fits in a multivitamin.

You can get sufficient potassium by eating 2-3 meals with plenty of vegetables or leafy greens every day, and you can get sufficient iron by eating animal meat (unless you have a diagnosed iron deficiency and need to supplement with it directly).