I have been able to upgrade a silverstripe installation to version 3.
I had to comment same multilanguage features in order to be able to do it and add a line that i saw in a forum in the _config.php.
The site is working ok but the admin area doesn´t loads and i just get a silverstripe logo in the middle of the screen.
By inspecting the source there should me more links to different areas on admin area.
To say the true i am starting moving same sites to worpress as it is usually very difficult to do any upgrade on silverstripe and a simple new version can place one site not operational.
The issue is that i have done same sites to same clients and i am afraid that when i upgrade them i will make their sites not operational and if i leave them as they are they can be attacked somehow.
I had the same problem upgrading. I fixed it by removing assets/_combinedfiles and letting silverstripe regenerate it. I did a dev/build?flush=1 too, and removed the old jsparty folder, don't know if either of those steps are needed.