The Sci-Fi solution
Giant telepot gates that connect Venus' Equator with Mars' poles, and vice versa.
A hot wind from Venus will blast the ice to water around the poles of Mars, and the two planets will share all their volatiles at an Earth-like norm.
Venus and Mars will both be windy and dry, compared to Earth, but in time the CO2 will react to carbonate and Bob's your Auntie, 3 habitable planets.
Now all I need is an industrial-sized cargo of Stargates...
