Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

It's missing an analysis of orbital stability. Look at co-orbital objects. I believe most of them are unstable when dealing with objects of similar mass, and the only known similar-mass example is the Janus-Epimetheus system of moons.

Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

I ditched the near-sun planet, and just managed to put in a Terra-like Garden at 0.79 and a standard thin-atmosphere 'garden' at 1.1 au. I get the feeling that making borderline Garden/Rock and Garden/Greenhouse planets is somewhat harder than making any of the three types (takes much more careful balancing acts).

The star is currently 0.77 Luminosity.

I was forced to change temperatures of the terran planet to 13/27/43°C. A bit on the high side, but I guess that just means I have to make the polar zones almost habitable, and the equator a somewhat uncomfortable place to live (from a human PoV, anyway). (For the record: I don't quite get how to calculate zonal temperatures from those three yearly world-average temperatures.)

Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

The ratio between the innermost and outermost orbital radii that can possibly Garden planets in the Space worldbuilding section is 1.76. That's enough to have a pair of Garden planets (as small a ratio as 1.4) but not three (minimum ratio between the inner and outer is 1.96), without resorting to moons.

Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

A rosette orbit could work, but that indicates deliberate placement.

That or make the furthest "inhabitable" planet a greenhouse world.

Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

Yeah, I didn't clarify that I meant a problem with three. You can get three by having two co-orbital and a third further out, though that probably reduces the stability of the co-orbital pair.

Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

Greenhouse worlds start at 500°K anyway, so it won't be.

Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

Not sure if this is GURPS-compatible or not (don't have Space) but have you considered using a gas giant?

The goldilocks zone for a typical star seems too small to allow multiple planets. But multiple moons might be doable, if you're willing to endure some weird scenery. (A double-planet satisfies this as well).

Another way to stretch the habitable band might be to have a much larger world with a much denser atmosphere as your outer planet. The greenhouse effect could buy you a little wiggle room that way.

Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

Does the goldilocks zone get wider if your star is really large and luminous? Obviously you'll have very long years, but that's bearable.

Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

I don't think that will help. It will result in this zone being further from the star, and the minimum distance between planets is defined through the ratio of their orbits.

This wouldn't be much of a problem if it was easier to adjust blackbody temperatures of planets, but those seem to depend heavily on planet types.

Re: [Space] Making a solar system with 3 (semi-) habitable planets?
 

Just to be clear, I was elaborating on your statement, rather than refuting.