Why does he want to terminate rights, to get out of paying child support? does he see any of the kids? This is a terrible idea if there is a parent-child relationship already established.
You would be taking on a huge responsibility, legally and financially, by doing this and a judge is going to be very reluctant to do this. Actually you cannot do this until you have married her, and likely the judge will want you married for a certain amount of time.
It is hard to terminate rights involving 1 child, let alone 3.
I would not let this bio dad off so easy. If he does not want to pay support, too bad, your fiancee should file for child support.. He does not have to see the kids if he chooses not too. Child support and visitation are 2 seperate things.
Has your fiancee moved so he is not able to see the kids? Why does she want to do this?
The interest of the kids comes first here, and it is an uphill battle to prove a judge termination of rights is in the best interest of the children.