I'm not sure what you mean by "optimized for use in browsers". I can't speak to the other 2, but Chromium is basically a vast collection of C++ cross-platform libraries that can be used in many applications.
The core Chromium codebase itself uses the same license (a BSD-style license). Chromium itself uses certain other third-party libraries that have different licenses.
Qt is a higher-level application dev framework with UI support (like MFC but cross-platform). We were looking for lower-level constructs - especially in the networking and P2P areas.