I am trying to find out who wrote the music that the boys were dancing to whenever there was a party/dance.I don't mean when Don Grady would sing..thanks

The composer of all those incidental music cues (especially during the color years) was Frank DeVol who not only wrote the theme to "My Three Sons", but also "Family Affair" and co-wrote the theme to "The Brady Bunch" with its creator Sherwood Schwartz. DeVol (as he was mainly credited) was born in 1911 and passed away in 1999.

Geoff, Don Fedderson had (in part being in on the Desilu lot) used the then-ubiquitous incidental cue music from Capitol Records's music library which in turn encompassed many British music cues. But DeVol (RIP since 1999) was there from the beginning, and he did indeed wind up doing incidental music. Of course he may have had composed the stock music as well freelance. (Cf. Hoyt Curtin at Hanna-Barbera-which also used that music).

(Frank) DeVol wasn;'t the only music director to use just his last name--(Tutti) Camarata wortked for Jimmy Dorsey and for Walt Disney (not to mention countless recording artists from DeVol and Camarata's contemporary Matovani to pop icon Madonna etc.etc.)