To add to @rjniles reply and while you'll need to establish 3 accounts with visible, depending on your existing carrier there may be some extraneous charges incurred if post paid and getting discounts for those additional lines.
In my case when switching from T-Mobile I incurred a slight hit going from 2 lines to 1 line - on completing the 1st port the remaining line's cost increased (discount for 2 lines was removed) and was prorated for until the remaining line was ported. Fortunately, it was only 1 day of proration even though only a few hours between ports. Had it been longer with porting problems it would have been a pretty substantial proration - something to consider proration even though only a few hours between ports. Had it been longer with porting problems in planning your transition. Activating and completing port close together will avoid some unexpected charges.
In addition I got dinged (prorated) for a couple of their prepaid promotions - Apple TV+ and Netflix On Us but am glad to have move on so it was worth it.
Probably not an issue if your existing multiline account is prepaid with no promotional discounts.