I am surprised I could not find this question already asked, so if I simply missed it please notify promptly.
I need to write a very small, fairly simple application in .
As other posters mentioned, BitRock InstallBuilder supports your target platforms of Windows, Linux and Mac. I want to add that it has been used by the Mono project itself for a while to distribute all-in-one binary installer for Mono runtime. It is also used by other successful Mono-based ISVs like Codice Software