During dark reaction the three carbon atoms of 3-PGA are derived from

During the dark reaction, the three carbon atoms of 3-PGA are derived from RuBP + CO₂.
Dark reactions of photosynthesis are a series of chemical reactions that convert carbon dioxide and hydrogen-carrier compounds into glucose. This is because the process requires NADPH, which is short-lived and comes from light-dependent reactions.
The CO₂ that is captured by enediol produces an unstable six-carbon compound called 2-carboxy 3-keto 1,5-biphosphoribotol that immediately splits into 2 molecules of 3-phosphoglycerate , a 3-carbon compound. Two 3-PGAs are produced for every CO₂ that enters the cycle, so this step utilizes two ATP per CO₂ fixed.