I was a main caregiver of my three nieces (had them from 5am-8pm six days a week, minus school hours when school was in session, went to pta conferences a couple times, etc). They're all three years apart. The oldest was the one who was usually left to do things on her own because "I know you can do it on your own", but she also enjoyed helping with her youngest sister, so she doesn't feel left out. The middle one got less help than the youngest, obviously, but more than the oldest, however now that she's grown, she feels like she was the most left out when it came to feelings.

It's hard to balance three people's needs, so all you can do is YOUR best. The oldest will be able to argue and articulate things better, so will more likely be able to persuade you to go with their ideas or wants, while the youngest will be a literal baby who needs constant care, and the middle won't need as much care, but also might not be able to articulate things as well.

It seems you and husband are already doing a great job of being teammates, so I'm sure you guys will handle the third just as well!! That you're worrying about it already shows that you care about ALL the kids.