Please note that Sitefinity 3.0 is not backwards compatible with neither of the previous versions. One difference is that it uses Master Pages, Themes and Membership services - features which are .NET2 dependent and which have not been provided with Sitefinity 2.7 for example. Also, Sitefinity 3.0 has completely new architecture, offers new layout rendering, i.e. it does not use tables any more as Sitefinity 2.7 does, but rather DIVs and CSS. This comes to say that unfortunately, you won't be able to upgrade your project created with Sitefinity 2.7 to version 3.0. You can reuse only your content but you should recreate your project again.
I hope that this information will help you. If you have more questions, do not hesitate to contact us.
All the best,
the Telerik team