Question: are apple watch series 1 bands compatible with series 3?
Answer: You can use a band that's designed for Apple Watch (1st generation) or Apple Watch Series 1, 2, and 3 with Apple Watch Series 4 and Apple Watch Series 5, as long as the sizes are compatible. Bands for 38mm and 40mm cases work with each other, and bands for 42mm and 44mm cases work with each other.