I have installed Qt5 libraries in windows but there is no document about Qt3D in Qt assistant. Is Qt3D a part of Qt5 or it has been removed from release version 5?
The other answer & comments were correct at the time, but the situation has now changed.
As of Qt5.5, Qt3D is now included as a "technology preview" (I interpret this phrase as an indication that it's not deployment-ready, so there could be some bugs and the API could change).
It's very important to note that the Qt3D that was available for previous versions of Qt has been Qt3D 1.0, while the version included with Qt5.5 is v2.0, which has been developed by a different software team and has a very different API.
More info from the new developers available here.
Note also that Qt3D v1.0 was never actually included 'properly' with Qt in any version, though was (apparently) reasonably easy to build along with Qt if checked out from git from v5.0 through 5.4.