The description of C3 hydrocarbon chemistry in current photochemical models of Titan's atmosphere is found to be far from complete. We have carefully investigated the photochemistry involving C3Hp compounds in the atmosphere of Titan (considering both photolysis and neutral reactions), which considerably impacts the abundances of many other hydrocarbon species (including C2 compounds). Model results indicate that three species (C3, c-C3H2 and C3H3) could be abundant enough to be present in the Cassini/INMS data. Because the error bars on predicted C3-hydrocarbon abundances are considerably larger than those of the observational data, new experimental and theoretical studies targeting the measurement of low-temperature reaction rate constants and product branching ratios are required to reduce current model uncertainties. In particular, we highlight 30 "key reactions", the uncertainty factors of which should be lowered to improve the quality of photochemical models involving C3Hp molecules.