Final answer:
Duke Ellington's sound was influenced by ragtime, blues, and jazz.
Step-by-step explanation:
The three principal sources of Duke Ellington's sound were ragtime, blues, and jazz. Ragtime influenced the syncopation and improvisation in Ellington's music, while blues contributed to the emotional depth and expression in his compositions. Jazz, being a uniquely American form of music, provided the foundation for Ellington's innovative arrangements, use of extended harmonies, and complex rhythms.