Flex/Air is probably as close as it will ever get to being write once/run everywhere.
As far as I can tell, Air is designed to only run on Linux systems with the Apt, RPM or YUM package managers. I had tried to set it up on Slackware using some workarounds (see http://cookbooks.adobe.com/post_Adobe_AIR_Application_Instal...), but never got it to work. The .bin installer would just produce cryptic error messages and fail.
I did succeed in installing Air by using rpm2tgz on the RPM package, but installing an Air application still fails.
Am I the only one with this experience?
Pretty much any modern language will run on all the operating systems you mentioned, without having to modify the code.
What you want is a framework/standard library to abstract away platform specifics. There are many alternatives out there but none of them delivers what they promise. One of the biggest problems is the lack of any standard GUI interface.
P.S. I think Javascript, HTML and CSS is a good combination but not what most people want.