D3, because definitely don't get enough sunlight for at least half the year. Magnesium, because in winter months we buy trucked-in produce that IMO is probably magnesium deficient.
I'd consider a few contenders for third. K2, which works synergistically with D3 and calcium; even though Primal probably delivers all the K2 needed, it might be good if your diet tends towards the veggie or pescetarian end of the Primal spectrum.
I also occasionally throw in mushroom supplements, even though I cook with shiitake, maitake, oyster, cremini, beech, and other mushrooms: a good blend of shiitake, maitake and reishi (NOT a good cooking shroom), or cordyceps sinensis. These are all-around tonics (adaptogens) that don't get much play in Primal discussions, but I think they enhance your immune response and are good anti-inflammatories. Cordyceps may be a good call if you are very athletic.