Everyone seems to call it just plain '3BSD': that is certainly what the people who did it called it - see e.g. Kirk McKusick here: "an enhanced version of 3BSD for the use of the DARPA community". (You will find the same thing in Peter H. Salus' book, "A Quarter Century of Unix".) The original documentation doesn't seem to include a short name; see e.g. Setting up the Third Berkeley Software Tape. (If you do Web searches for "3.0BSD" and "3.0 BSD", almost all the hits are for BMW car parts.)
The 'BSD X.Y' form did not really arrive until 4.1BSD (see here: "Rather than continue shipping 4BSD, the tuned-up system ... was released as 4.1BSD in June, 1981"). So, I'm going to move it back to the '3BSD' name.