Unfortunately, at this time I don‘t know a source of three year old Paeonia parnassica.
I received my plant from Vlastimil Pilous from Czech Republic. But in the last two years he offered wrong plants, which are Paeonia mascula ssp. hellenica, which also grow at the same location on Mt. Parnass.
The only Person, I know that offered Paeonia parnassica at this time is Jan Moers from Belgium. He offered 1-year old plants last year.
All of my Friends, which had Paeonia parnassica in their garden, lost them. But my plant grows very well in pot. And my young plant too.