Turn the question around: To what extent is it possible to terraform multiple planets in a solar system? If you can come up with plausible scenarios for that, you may be able to also have situations that arose that way naturally.
Unfortunately I have no experience in terraforming. You may want to consult someone experienced.
As @kingledion comments, Mars had running water.
Venus has a greenhouse effect run wild. Had Venus been in Mars orbit it might be quite habitable. This seems reasonable. Mars is a bit of an oddball, being smaller than the earth. As you get further out, the gradient of the gravitational field decreases, so a protoplanet should be able to capture from a larger slice of their lane in the ecliptic. Mind you, the planet boffins have found jupiter sized worlds in Mercury sized orbits, so my mental model of planet formation is clearly lacking. Being fallible sucks.
Similarly suppose a planet at Venus's distance early on had a companion planet or large close moon that broke up into a fairly massive set of rings. Would this cast enough shade to reduce the temperatures to a reasonable level.
Another way to get more habital planets is to stack the orbits closer together. This may not work. See the wikipedia article on Titus-Bode's law. Too close together and their orbits become unstable. Play with it a planetary similator maybe?
How about double planets? They would be tidally locked. See Roche's limit for how close. If you had a companion for earth, the limit for small co-orbiters is about 40,000 km. Larger ones I think will increase this. But suppose you have an earth twin 80,000 km (50K miles) away. From the surface of either planet the other one is 1/3 the distance of the moon, and 4 times the diameter, giving an object that subtends 6 degrees of sky, has 36 times the area, and since the earth is a lot brighter than the charcoal grey of the lunar surface, several hundred times the brightness at night.
Suppose you have a planet that is marginally too close to the sun. The equatorial regions are lifeless deserts. You have the possibility of two entirely separate genesises (genesi?) with different chemistry.