Without knowing the city that you refer to as XXX it is impossible to give you an accurate answer. If XXX requires a certain minimum stay, etc. then the answer is different than if that is not the case.

Also, you don't NEED to book 3 round trip tickets then you separately from XXX to MSP to XXX. You can book the other two in one reservation, you in one reservation for all FOUR flights then fly with them. Being in one reservation alone does not require UM status - only if an adult is not flying with them. I know this to be a fact and have had friends do just that. The only differences are (1) if you book your ticket as one ticket for all four legs, you have no issues at all if DL prices it like you expect, (2) most likely you save a few dollars in taxes.