C3 plants involve direct carbon fixation of CO2. That is, the initial steps involve the CO2 being bound to ribulose bisphosphate to produce two molecules of three-carbon compound (i.e. 3-phosphogylycerate). The key enzyme that catalyzes carbon fixation is rubisco.
C3 plants must however be in areas where CO2 concentration is high, temperature and light intensity are moderate, and ground water is abundant. This is because in hot areas, the stomata are closed to prevent water loss. However, it results in the rise of O2 level. When this occurs, rubisco reacts with O2 instead of CO2, and leads to photorespiration, which in turn, causes wasteful loss of CO2 in C3 plants.