So to answer your questions -
1. You can book 3 tickets from your account itself. Transfer all your points to your account and add family members as redemption nominees. Ideally you cannot transfer your credit card points to anyone's krisflyer account except your own.
2. There is a way to transfer miles from a child's account if the child has miles gathered (have not tried this yet) but that requires the child's accounts to be linked.. attaching screenshot of that . Do not think there's a way to club your spouses points(incase it has miles gathered) with yours.
View attachment 19223
3. There is an option to pay the difference when miles fall short -
https://www.singaporeair.com/en_UK/us/ppsclub-krisflyer/use-miles/mix-miles-and-cash/
This is just from my one time experience of redeeming miles for a one way ticket from SIN to MUM for a family of 4, had 80000 miles so 4 tickets was possible from one account itself, only had to pay taxes. There could be other ways I'm not aware of, which our forum's experts will surely guide you.