You can always call and ask. You might get a reservations agent who is willing to put all three of you in Economy Plus to save the Gate Agent the headache of trying to get you three seats together day-of-flight.

However, note that if for any reason the seatmaps get re-swept (flight change, aircraft change, etc.), the computers will cancel any E+ seating for unqualified (non-UA/Star Elite) passangers. So you will need to keep an eye on your seat assignments.