The best way is to assign an adult AppleID as the family organizer and create a family share iCloud account.
Have the the other two create new AppleIDs using preexisting email accounts.
Once they are created sign out of the previous Apple ID on the devices. When they sign out they’ll be prompted with messages that ask if they want to keep or erase the iCloud data on the phone. KEEP THE DATA. Once the new Apple IDs are signed in, make sure all of the content they want backed up is selected in the iCloud settings, then do a back up. After the back up have the family organizer (the main Apple ID that’s currently being used) invite the two newly created AppleIDs to the family. Once the invitations are accepted you can choose to share purchases, music, contacts, location etc.
The best advice I can offer is to read the prompts on your devices carefully before choosing a response and see the articles Apple has available about family share, transferring iCloud data and creating Apple IDs.