The nephews never having heard of the Three Caballeros might be a reference to the fact that the film The Three Caballeros (1944) is one of the more obscure Disney classics.
The Three Caballeros (1944) was a 1944 Disney feature that combined animation and live action. It depicted Donald visiting Brazil and Mexico with Jose and Panchito acting as tour guides to their respective countries. It, along with its predecessor Saludos Amigos (1942), was produced as part of America's "Good Neighbor Policy" to promote friendship with Central and South America during the Second World War.
A banner can be seen reading "ATE MAIS," which is Portuguese for "Farewell."
In his original appearance in The Three Caballeros (1944), Panchito had pistols in his holsters which he fired into the air in excitement. However due to sensitivity issues as guns have become banned from children's entertainment, much like with other recent Disney productions involving Panchito, Ducktales does similar censorship by replacing the pistols with Smartphones that are held in his holsters instead.
Panchito greets the kids with "saludos, amigos." This is also the title of the Disney animated film which introduced Jose Carioca, Saludos Amigos (1942).