Not even full 3.0 as zombie said, it started around 2006/2007, its a mixture of UE 2.5 and 3.0. Furthermore, If you even read the classes inside Engine.u, there are some classes that state on the first lines of them "//copied from UT2004" which is a sequel/expansion of UT2003, that started to be produced in 2002...
So yea, it has pieces of code much older than people think, that's why engine upgrade at this time is mandatory. And yes, i actually compared those classes with UT2004 ones just to see if that comment was some sort of inner joke between devs, but it wasn't, those classes are pretty much copy and paste to APB