Hi !
I'm a violist, and I'd suggest you check out the Suite No. 3, op. 19 by Kurt Atterberg. Very national romantic music, but beautiful, pleasant to play and quite original after all.
Here's a good recording of the 1st movement :
Although it was originally written for violin, viola and chamber orchestra, I've played the reduction for piano and it sounds good

Max Reger has written a piano trio for violin and viola. It's nice as well! (op. 2 in b minor)
Or you have the Eight Pieces for Violin, Viola and Piano in C minor, Op. 83 by Max Bruch, or the trio in c-minor by Mendelssohn Bartholdy, trios by Brahms, Mozart, Contrasts by Bartók.... These are however all examples of the many adaptations from various cello/viola/violin/clarinet constellations, so not originally written for piano, violin and viola.
Hope you find something you enjoy !
Ane
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