reply to Ponytales
You can do what's called an "Assumption of Liability" on your three lines, which will start a new family share plan under your name. Those three lines would continue under contract, and you'd then be liable for all monthly charges. Then, you could add a new (fourth) line to your family share plan.
There are no fees involved. Just have the current account owner call 611 to authorize you to take over the three lines. You would then call the Assumption of Liability 888 number to accept the transfer and open a new account.