Among the many works written for piano, violin, and cello, the Beethoven Triple Concerto holds a unique place in the canon as a veritable concerto for trio — and we would be hard-pressed to find a trio more capable of meeting its challenges rhan pianist Jeremy Denk, cellist Steven Isserlis, and violinist Joshua Bell, three extraordinary alumni of the Aspen Music Festival and School who return to perform it as part of the festival's 75th anniversary season! We sat down with them to hear about their long history of celebrated collaborations, what they're looking forward to as they prepare this repertoire gem together for the first time, and which instrument has the most difficult role to play in Beethoven's score…