The three main external factors that affect the rate of photosynthesis are light, temperature, and carbon dioxide concentration. The rate increases with brighter light and higher temperatures up to a point. More carbon dioxide also increases the rate up to the saturation point. These three factors are considered limiting factors, meaning the rate is limited by whichever factor is least favorable. Internal factors like chlorophyll levels and enzyme activity also influence the rate. C3 and C4 plants differ in their responses to light, temperature, and carbon dioxide levels, with C4 plants having an advantage in higher temperatures but C3 plants benefiting more from increased carbon dioxide.