C3 Photosynthesis. Plants which use only the Calvin cycle for fixing the carbon dioxide from the air are known as C3 plants. In the first step of the cycle CO 2 reacts with RuBP to produce two 3-carbon molecules of 3-phosphoglyceric acid (3-PGA).4 plants almost never saturate with light and under hot, dry conditions much outperform C3 plants. They use a two-stage process were CO 2 is fixed in thin-walled mesophyll cells to form a 4-carbon intermediate, typically malate (malic acid).