To upgrade Forms 3.0 to 6i you will need to upgrade them to 4.5 first, and then to 6i since direct migration could not work.
To perform the upgrade to 4.5 you will need the Forms 4.5 software.
From the documentation:
“To upgrade a pre-4.0 form to version 4.5, type this statement at the command line, substituting the correct form of the Forms 4.5 Generate command for your environment:
f45gen32 <module_name> <username>/<password> upgrade=yes version=<version_number>"
For forms 3.0 you should use version number 30
See chapter 16 of this document.