Are multivitamins / supplements a complete meme or do they at least somewhat help makeup for certain deficiencies in your diet? I'm workshopping a 1800 calorie diet and I don't think I'll be able to fully cover Omega 3s, Calcium, Magnesium, Zinc and B12s.

At best they can't hurt. You should innately know if you're eating decently enough or not to begin with.