Why does NADH produce 3 ATP while QH2 and FADH2 only produce 2 ATP?

I learned in my biochemistry class that for every one molecule of NADH give rise to 3 ATP while QH2 (ubiquinol) and FADH2 only produce 2 ATP. Why is this? What process does each molecule have to go through to give rise to the ATP they yield? Thanks!
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