Jeremiah Was a Bullfrog/Joy to the World

This was sung on some special on which the Brady Bunch appeared around 1974. I don't believe it was an episode of the series and am sure it was before the variety show appeared. Bobby/Mike soloed and the others backed him up on "Jeremiah Was a Bullfrog/Joy to the World" by Three Dog Night.

Can anyone tell me which show this was and whether it's available anywhere? Also was the song ever released on a record, if so which one, and might the record ever be released on a CD? Or might there be a CD of all the Brady Bunch kids' solos and might this be included? Thanks.