I would like to customize AutoCAD Plant 3D for my company needs. Is there any API for this? How can get it?
A good point to start with is the page on the Autodesk Developer Center, describing the API for AutoCAD P&ID and AutoCAD Plant 3D:
The API for these two products is called Plant SDK and can be downloaded from the page mentioned above. It includes documentation, code samples, include files and libraries for C++ development and .NET libraries.