I have seen a Lockheed Jetstar that was operated by the FAA (USA) with reg N1 how is this entered in plane logger, when it says it must be least 3 charactors

For aircraft with a registration of less than 3 characters, or containing symbols (such as + etc for German military aircraft) you will need to browse to the aircraft first to add the sighting.

In your case, use the search bar at the top to search for operator, and search for "FAA". Click on the FAA listing, then click historical records for Lockheed Jetstars. locate N1, and click View. Click "Add Sighting" on the aircraft page, and you will now be able to add it.