Because you have all three lines in unlimited, there are only three ways to get something fairly close to keeping your unlimited data. Either...
1. one of those lines would have to become a tiered line. If there is any one of you that uses minimal data, then move the upgrade to that line, activate the new phone on that line and choose a tiered data plan for that line. Then once the new phone is activated on that line and the contract updated, you can switch the new phone to any line you want.
2. Add a fourth line, buy the new phone fit that fourth line, choose a tiered plan for that fourth line. Once activated, you can switch the new phone to any line, put a dumb phone on that line and cancel the tiered data but would still cost an extra 9.99 for the next two years for the fourth line.
3. Buy a phone full retail, used or eBay, swappa, etc and can put that phone on any line.
1. one of those lines would have to become a tiered line. If there is any one of you that uses minimal data, then move the upgrade to that line, activate the new phone on that line and choose a tiered data plan for that line. Then once the new phone is activated on that line and the contract updated, you can switch the new phone to any line you want.
2. Add a fourth line, buy the new phone fit that fourth line, choose a tiered plan for that fourth line. Once activated, you can switch the new phone to any line, put a dumb phone on that line and cancel the tiered data but would still cost an extra 9.99 for the next two years for the fourth line.
3. Buy a phone full retail, used or eBay, swappa, etc and can put that phone on any line.