The trick is populating the 3.0 database. I know there are data pump tools out there but they seem to be old?

Use "Extract Metadata" feauture of IBExpert with table data extraction

My question:
 Now that my 1.56 metadata script works w/out error with the 3.0 database, can I simply backup the 1.56 database and restore under FB 3?

Try. If does not work - migrate to FB 2.0 using corresponding tutorial (if any), then to FB 2.5.9, then to FB 3.

E.g., backup DB with FB 1.5 -> restore with FB 2.0 -> backup with FB 2.0 -> restore with FB 2.5 -> backup with FB 2.5 -> restore with FB 3

At least 2.5 to 3.0 works perfect. Just download archives with required versions of FB (you need 'bin' directory with gbak.exe), and create bat-script with commands.