The third stage rocket provides the velocity to escape earth's gravitational pull. During the second stage firing, the spacecraft is only going about 19,500 mph and can't escape the earth's pull. The third stage boosts the speed to roughly 25,000 mph (escape velocity) and frees the spacecraft to escape from Earth orbit.
To reach escape velocity, the solid rocket motor will burn for about 90-seconds for both Rover A (spirit) and Rover B (opportunity), using approximately 2,020 kilograms (4,453 pounds) of solid propellant. This propellant is composed of ammonium per chlorate and aluminum.
After the spacecraft is on its way to Mars, it needs to spin fast enough to stay stable, but not so fast that the navigators would later need to use too much fuel to turn the spacecraft for correction maneuvers. The spacecraft uses its own propulsion system to slow the spin down further to 2 rpm.