In the final scene of Mozart's "Don Giovanni", the title character is being serenaded by an onstage band. They play three tunes. What are these numbers, from which other operas were they quoted, who wrote them, and what is the original context they are sung in?

1st, from Martin y Soler's "Una cosa rara"

The opera was first performed at the Burgtheater, Vienna on 17 November 1786. It was a huge success. Mozart quotes the music to the ensemble O quanto in sì bel giubilo towards the end of Don Giovanni.

https://en.wikipedia.org/wiki/Una_cosa_rara

2nd, from Giuseppi Sarti's "Fra i due litiganti il terzo gode"

Fra i due litiganti il terzo gode ("While Two Dispute, the Third Enjoys") is a dramma giocoso in two acts by Giuseppe Sarti. The libretto was after Carlo Goldoni's Le nozze (The Marriage).
One aria from this opera, Come un agnello, is famously quoted by Mozart at the end of Don Giovanni.

https://en.wikipedia.org/wiki/Fra_i_due_litiganti_il_terzo_gode

3rd, from Mozart's own "Marriage of Figaro".

Don Giovanni's chambers

Giovanni revels in the luxury of a great meal and musical entertainment (during which the orchestra plays then-contemporary late 18th century music – including a reference to the aria "Non più andrai" from Mozart's own Le nozze di Figaro).
This is Figaro's aria, telling Cherubino that his days of philandering are over.

https://en.wikipedia.org/wiki/Don_Giovanni