It's 3. Not 5. The IMM008 is for applicants applying for PR through PGP. You list your spouse and any kids of your own in the IMM5768 financial evaluation form to total 5.
Thanks. Sorry I did not look at the form. But the OP asked "what should I put down for question 3, How many family members,, including you, are in this application? " so I guess he read it wrong too.