Hi ,
Sweeney has already commented on the potential for UE3 open source on this link: Open Source UE3/UE2 under same license term as UE4? - Feedback & Requests - Epic Developer Community Forums. His response is below:
"We would like to, but can’t. We have investigated this possibility extensively over the years and concluded that releasing UE2/3 source was infeasible due to a combination of third-party licensing contracts, lack of source, and compiler incompatibilities.
The UE4 code release now has been made possible by an extensive architectural effort to eliminate problematic third-party code dependencies which began back in 2010."