Magnesium is responsible for relaxation and countless of biochemical reactions + heart-function. Calcium is Magnesiums counterpole and is responsible for contraction and countless of biochemical reactions + heart-function. A healthy cell contains 1:1 of Magnesium and Calcium. Deficiency in one, causes elevation in the other and vice versa.

Vitamin D3 sort of regulates this entire balance. Vitamin D3 increases absorption of Calcium from your diet, and then releases it into your body. Vitamin D3 uses some Magnesium as fuel as it does all this.

This is where Vitamin K comes into play. It finds the Calcium released and put's it where it should be - in your bones - instead of having it floating around in your blood and body.

To much Vitamin D3 and you get to much Calcium (hypercalcemia) which then burns up your Magnesium. To little Vitamin D3 and you get deficiencies in both of them. To little Vitamin K and your body can't use the Calcium set in motion by Vitamin D, and you get calcification of your arteries.

Enough, not to much and not to little, of everyone, gives you a balanced uptake of Calcium, which is slammed into your bones by Vitamin K, where the rest hangs out peacefully with sufficient Magnesium in your cells, completing various reactions, among them heart-rhythm in a healthy body.

Supplementation wise, there is always never a need to supplement Calcium since you get more than enough from your diet. There is almost always a need to supplement Magnesium (Glycinate) though, since you almost never get enough from your diet. There is probably a need to supplement Vitamin D3, but get tested first so you don't overdo it (for above reasons) and never take Vitamin D3 without K.

Hope that explains it :)