Try looking here for previous versions of MacRuby.
I want to say that version 0.10 was compatible with Snow Leopard, but I'm not certain.
Question
Is it possible to run MacRuby with XCode 3?
It is not possible for me to upgrade to XCode 4.2 because I am running Snow Leopard and have a machine that will not upgrade to Mountain Lion and Apple has pulled Lion from the App Store. I know you can download XCode 4.2 with a Apple Dev Account ($100 per year) but I am looking for a free solution.
Maybe there is an older version on MacRuby that can be installed with XCode 3? If you know which version, I would be interested if the installation instructions differ from that of XCode 4.2
Solution
Try looking here for previous versions of MacRuby.
I want to say that version 0.10 was compatible with Snow Leopard, but I'm not certain.