OK what she don't file until the baby is 3 yrs old then do I have to pay all that back pay from she was born?

First, its not a good idea to piggyback your question on another thread. To avoid confusion your post has been moved to its own thread.

Second, any question on law needs to include your general locale since laws vary by area.

But generally, support goes back to when the custodial parent submits the court petition. However, some areas do allow some level of retroactive support.