I *think* that if you were to temporarily turn auto-aging off, that you would be able to set birthdays and age up your three boys at your leisure.
I generally play with aging off and have birthdays when it’s time to move on to the next stage, because I like to complete a lot with my sim kids and I don’t want to have to race the clock so to speak in order to complete my goals for them. Perhaps the same sort of set up could work for you? And once everyone is aged up at the right time, you could turn aging on again and business would resume as usual. It might even give them a different birthday on the calendar for future, depending on how that feature works.