C3 carbon fixation is a pathway for carbon fixation in photosynthesis. This process converts carbon dioxide and ribulose bisphosphate (RuBP, a 6-carbon sugar) into phosphoglycerate through the following reaction:

6 CO₂ + 6 RuBP → 12 phosphoglycerate

This reaction occurs in all plants as the first step of the Calvin cycle. In C4 plants, carbon dioxide is drawn out of malate and into this reaction rather than directly from the air.

Plants that survive solely on C3 fixation tend to thrive in areas where light intensity is moderate, temperatures are moderate, carbon dioxide concentrations are around 200 ppm or higher, and ground water is plentiful.