There is an episode guide for the Smothers Brothers at https://epguides.com/SmothersBrothersComedyHour/

Season 1 had 20 episodes, from 5 Feb 67 to 25 June 67.

Season 2 had 26 episodes, from 10 Sept 67 to 10 March 68. Seeger sang Big Muddy on episode 24, 25 Feb 68.

Season 3 had 26 episodes, from 29 Sept 68 to 20 April 69.

All of that was on CBS. The brothers came back on NBC for 13 episodes in 1975, from 13 Jan to 19 May. Details at https://ctva.biz/US/MusicVariety/SmothersBrothersShow_1975.htm