GeneXus 15 Upgrade 3 previews were part of the releasing process of Upgrade 3. They were intended to round up Upgrade 3 and facilitate adoption once released.
GeneXus: http://www.genexus.com/developers/RN?,0,,0;4;V15;2;3;;
GXserver: http://www.genexus.com/developers/RN?,0,,0;6;V15;2;3;;
GXflow: http://www.genexus.com/developers/RN?,0,,0;3;V15;2;3;;