It is possible that some dedicated 3D graphics converters support 3dp to obj conversion, but such conversion does not make much sense as 3DP files generally contains photos (with 3D effect) and OBJ is a format for storing 3D model data.
Unfortunately, we do not have information about any specifics, so if you have issues with .3dp files, it would be perhaps best to contact Autodesk support.
Open 3DP file Open OBJ file3D viewer and converter with a vast file type support