A project has seed file of 2D of default design model, default sheet model & DPF sheet model. How to convert all these 2D seed to 3D in MX Road SS4 V8i?
That depends on the configuration - from your earlier post, it looks like you used 'Bentley-Civil-Imperial' project, so it reads them from C:\ProgramData\Bentley\PowerInRoads V8i (SELECTseries 4)\WorkSpace\Projects\Examples\Bentley-Civil-Imperial\dgnlib\
This is a separate topic to seed files, and should be in a different thread.
Frank - you do not seem to understand the idea of Feature Definitions [FD]. They are in the DGNLIB - a library, not a seed file. Why are you trying to make a FD 3D DGNLIB - it makes no sense.
When you create a new file File > New, you can pick a 2D seed file or a 3D seed file in the dialog, and your file is 2D or 3D. The seed files are in a 'seed' folder - check your folder structure.
The FD will be read from the DGNLIB as needed - they have nothing to do with 2D/3D..
Frank - I do not understand your comment - if you create a 3D file, you will still have the feature definitions in the workspace library, so they will still be available. What software are you using?