I’m really confused? 
I have hundreds of Halion 3 fxb files and they all load into Halion 6 perfectly and have never had to convert them - they just work!
I don’t understand why you’re all having a problem?
Can you see a picture of Halion 3 under the list of instruments in the Halion 6 medibay and if not that may be the issue, but I’m not sure?
Looking forward to your reply.
Kind regards
James Colah