My multivitamin has 120 mg of Magnesium, and I am taking 500 mg of Niacin/B3.
you should be on 600-800 mg of mag. seems like a lot but it’s not. also, chances are, the form in your multi does not absorb so it’s essentially fruitless.
Is your B3 niacin or niacinimide? If the latter, it’s the non flushing form and your should be on the former. Personally, I take both forms but I’m on 500mg of niacin (flushing kind) 2x per day