Soybean (Glycine max L. Merr. cv. Bragg), peanut (Arachis hypogaea L. cv. Georgia Green) and rice (Oryza sativa L. cv. IR-72) were grown for a full season in sunlit, controlled-environment chambers at 350 (ambient) and 700 (double-ambient, elevated) μmol CO₂ mol^-1 air, and under daytime maximum/nighttime minimum air temperature regimes ranging from 28/18 to 48/38^oC for soybean and peanut, or soil water deficit for rice. The objectives were to characterize the interactive effects of elevated [CO₂] and high air temperature, or elevated growth [CO₂] and soil water deficit, on leaf CO₂ exchange rate (CER) and Rubisco activity, and to test whether elevated [CO₂], high temperature or severe drought stress would induce changes in the kinetic behavior [K_m(CO₂)] of Rubisco. Leaf CER of soybean, peanut and rice were increased by CO₂ enrichment, but decreased by high temperature and drought.  For soybean, despite the deleterious effect of high temperature on CER, CO₂-enriched plants not only outperformed ambient-CO₂ plants at the optimum growth temperature (32/22^oC) for photosynthesis, but also compensated much better for the adverse effects of high temperatures on CER.  In addition, the degree of enhancement induced by elevated [CO₂] on soybean CER increased in a linear manner with increased growth temperature. For peanut, however, the degree of enhancement in CER by elevated [CO₂] as a function of growth temperature differed from that for soybean. For rice subjected to drought stress, elevated [CO₂] delayed by one day the substantial reduction in midday leaf CER. Elevated [CO₂], high temperature and drought reduced the initial (non-activated) and total (HCO₃^-/Mg²⁺-activated) activities as well as the activation state of midday-sampled leaf Rubisco.  The Rubisco initial K_m(CO₂) of soybean and peanut was not markedly altered by elevated [CO₂] or high temperature, as neither severe drought imposed at panicle initiation changed the initial K_m(CO₂) value of the enzyme for rice.  However, moderate increases in Rubisco total K_m(CO₂) were observed for soybean and peanut at elevated [CO₂] and high temperature. The moderate increases in Rubisco total K_m(CO₂) suggest that multiple generations of growth under CO₂ enrichment might be required for Rubisco of the C₃ species to evolve towards a more effective type enzyme.