When the version known as 3.5 started development it was originally going to be 3.1, but as time went on and new features were added the authors decided that too much was added for it to be just a .1 release so they upped the version number to 3.5 to help show that a lot more work had gone into it.
The 3.5 line of code is now up to 3.5.1 and has been out of beta for a long time. Your only concern will be whether or not the extensions you use will be compatible with the new version. As for upgrading you can just install right over the top of 3.0.12 and everything will be fine. You don't have to delete your old version first.
I just thought the rule was that no major new version was to be offered through auto-update just like one couldn't auto-update to v3 from v2, but v2.x upgrades were still offered to v2.x users.