Besides the Schnittke and Tippett (I also seem to recall a rather
well-known Vivaldi work, and there's a Mozart fragment), there's a
Weingartner sinfonietta for string trio and small orchestra (not a
concerto- can we sneak this one in...), a fantasia for string trio &
orchestra by Patrick Soluri, Rontgen's concerto for string trio and
orchestra, Vazgen Muradian's concerto for string trio and orchestra op.
154 (c.  1998) ...

-Eric Schissel (ok, only two of these are concerti.  sorry.  Lots and lots
of piano trio/orchestra concerti, not so many with string trio concertante,
near as I can tell...)