Steve Schwartz wrote:

>>Also, I can't think of a string trio by anybody that's well known, even
>>among classical aficionados.  Piano trios, yes.  String trios, no.

And Jocelyn Wang wrote:

>I've also noticed that, once a composer starts writing quartets, they
>seldom return to string trios.

While it's not called a string trio, Mozart's Divertimento in Eb, K563,
for violin, viola, and cello, a sublime work of music, was written after
Mozart had written many of his quartets and quintets (if K numbers are
an indication).

Walter Meyer