In fact, for only one more person, the three of you could simply show up for a 2-person ADR most places without an issue. Two are often seated at a table for four in our experience, and the weirdness of Disney tech makes it easier to get an ADR for 2 or 4 than for an odd number.
Try booking for 3 if you'd rather, but I truly don't think you need concern yourself about doing so.