Some changes to the Solar System provides us with three habitable world's.
Mars / Moon / Moon
Mars is in a closer orbit coupled with a full-sized moon and a diminutive asteroid as well as the positive effects of an active magnetic core have made incredible changes to Mars. No longer a dying world the red planet is still red, but green and blue also invade the surface. With a gravity about half that of Earth's, and a atmosphere roughly comparable to our own Mars is a lush planet with large Iron-rich deserts, and the solar system's largest active super volcano.
Mars itself has an day/night cycle of 25.75 hours, nearly identical to that of the Earth.
Earth / Moon
- Same as modern day
Venus / rings / Moon
The twin of Earth, Venus is the fourth planet from the sun and is very temperate - a little warmer then Earth because of the more robust greenhouse gas effect, but still quite habitable.
Scientists believe that the ring system of Venus was formed after two Moon-sized object collided hundreds of millions of years ago. The asteroid debree couldn’t reform into a moon, and instead became trapped in a circularized orbit around the planet Venus. Venus is the only Terrestrial planet in our solar system to have developed rings like the outer gas giants, and even though the rings have consolidated their orbit at an equatorial 125 km circular orbit they are visible from almost everywhere on the surface of the planet.
Venus itself has a uninhabitable moon, approximately the same size as our own Moon. This moon has over the eons stabilized the rotation of Venus, and allowed an 28 hour "day." Venus is a wet jungle world, drier then Earth but wetter then Mars or Vega. Scientists have calculated that 65% of Venus is covered by water, and that ninety-three percent of the water on Venus is salt water. Air pressure and Gravity are both approximately 85% that of "normal sea-level" conditions on Earth.
Vega / captured asteroids
Vega is the Mars analogue planet, dead or dying - just on the border of the inner habitable zone, and close enough to the asteroid belt to guarantee the capture of a number of asteroids from the main belt Vega is about as inhospitable as they come. Vega has no active core, and it has a surface water percentage of 15% liquid water on its surface which is quickly evaporating along with the decaying atmosphere.
This would have made the space race a whole lot more interesting, in fact if one of the three had remained primitive, then I can see that as the target for a space race, which keeps the people's attention.