Wolfgang Amadeus Mozart: Concerto No.7 in F for 3 Pianos K242. This is a 4-piano score. Only one score is provided (with Mozart cadenzas).
Mozart composed this concerto in 1776 for the Countess Antonia Lodron and her two daughters, Aloisia and Josepha (who were students of both Mozart pere and fils). The Lodron family served the Salzburg court throughout the 17th and 18th centuries. Moreover, the Countess, a personal friend of the Archbishop Colloredo, became first lady of the court during the years Mozart was active in Salzburg. The Lodrons and the Mozarts became neighbors in 1773 when the Mozarts moved into the famous Dancing-Master's House (the apartment boasted eight rooms and a dance hall!), and shared many musical events together. In fact, when strife arose later between Wolfgang and the Archbishop, the Lodrons suppported the Mozarts, and the Countess was instrumental in securing Wolfgang's return to Salzburg in 1778. As for the concerto itself, much has been said of the fact that the third piano part is not up to the virtuosic level of the other two (it was, after all, written for the eleven-year-old Josepha!). But while it may not make quite the same technical demands, it is certainly on the same level as the first and second piano parts in terms of invention (Mozart having taken care to sustain his young pupil's interest!).
[N.B.: This concerto also exists in a supposedly authentic version for two pianos; Maria Anna Mozart states in her diary that she and her brother Wolfgang performed it in the Mirabell Palace in Salzburg on September 3rd, 1780 (the present edition includes this version, as an insert; piano parts only).]