First - congratulations on the visa 309.
Legally married for 3 years with a child from that relationship.....met the Visa 100 requirement.
Perhaps there is another reason for the 309 visa being granted that the CO could explain for you.....?
They do make mistakes but you were expecting the Visa 100.....so there must be another reasons.
From immigration website:
There are four main criteria for the grant of a Partner (Permanent) visa.
1. Demonstrating you are living in a genuine and continuing relationship
(married or de facto) with your partner.
2. Assessing eligibility of any children who are also included in the Partner
(Permanent) visa application and that relevant custody criteria are met.
3. Character checks.
4. Health checks
From the Kingdom of Cambodia to Australia.......