[Buckeye30]

Nearly all the Nieuport trainers ( both single- and 2-seater) at USAS training fields in 1917-18 had a designator ---"3-" next to the field number on the fuselage and in the same style; a bit of a mystery unless it refers to the school, most going to Issoudon.

The field numbers were 900s and 1000s. Serials were not in the usual system, this is N10591, others included 10541 (930) and 10572 (958).
The table is from James Fahey's "U.S.ARMY AIRCRAFT 1908-1946" it shows the Nieuport 80/81/83 trainers aquired from France; "E" = Ecole (school) and "2" a 2-seater. In the last column "XIV" is the Type in this case "Training, air cooled".
A * shows the squadrons in service at the Armisice.
The N80/81 had the enlarged wing area ( 23m2 must be Nie.12 equivalent) and N83 had the 18m2 wing ( so Nie.10 presumably).
I wondered whether you had any thoughts on the "3-" ??