Arc3 is not released yet (it was just a joke, because we are waiting for it). Arc2c is a non-official, still in development version of a stand-alone compiler from arc to C. The official version (arc2, without the c) is available here : http://arclanguage.org/install . It was released at the end of February.
You can also download Anarki, an unofficial improvement of arc2 (mostly bug fixes, a few experimental ideas and a few more libraries), available here http://github.com/nex3/arc/tarball/master .
Personally, I think you really should use Anarki as it is more mature, but is still compatible with the official arc2 (and most people here would tell you so), but it's up to you.
