Whilst some dedicated 3D graphics converters probably support 3DP to STEP conversion, practially this conversion does not make much sense as 3DP files generally contains photos (with 3D effect) and STEP is a format for storing 3D design data about some part.
Convert Autodesk 123D photo scene file to ISO-10303 STEP product data.
We found one software available in the database for this conversion.
Microsoft Windows software - convert 3dp to step on Windows
3D viewer and converter with huge file type support
Convert from 3dp file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.