We would like to use Appcelerator Titanium to develop an IOS, Android and Blackberry OS 5.x app. I know IOS and Android are supported but is BBerry OS 5.x also supported as there's no mention of it in the docs?
There have been various preview releases over the last several years of BlackBerry support in Titanium. They've all been withdrawn, except for the most recent one, BlackBerry 10. You could hunt around the internet and perhaps find an old copy of one of the old versions (it was only released temporarily to customers), but it wouldn't be "supported", as your question stipulates it must be.
BlackBerry 10 is the only one in preview availability. The source has been released, and you can read about it on their blog: http://developer.appcelerator.com/blog/2012/12/announcing-preview-availability-of-the-blackberry-sdk.html