Hi Volkan, Artisteer 3.0 is currently a "Preview" edition, so it's not officially supported by mojoPortal yet. Joe has said that he will make it a top priority to support Artisteer 3.0 in mojoPortal once it's officially released. According to Artisteer's site, 3.0 is supposed to be released before the end of the year.
For the time being, I'd recommend you stay with Artisteer 2.5 for any production work. However, if you can identify the changes in mojoPortal that will be needed to support Artisteer 3.0 (new CSS classes, etc.) I'm sure that might help speed up Joe's work in supporting the final 3.0 release.