Robbie goes away at the end of season 11 to start a spinoff.

Anybody know what it was supposed to be about and why it failed?

Here is what "TV Guy" wrote in this forum way back in 2001:

"The last episode of the 11th season was the pilot for a proposed spinoff with Robbie and Katie living with the twins in SF. The spinoff was not picked up, and Don Grady chose not to return to M3S. So in the first episode of the 12th season, it was explained that Robbie had been temporarily transferred to South America, and Katie moved back in with his family while he was away, so they could help her out with the triplets."

By all accounts, the reason Don Grady did not return to M3S for Season 12 is because he decided he wanted instead to concentrate on his music career. He had been involved with music all through the seasons, co-stars have mentioned his practicing being heard off-camera at times and that he was skilled at playing many instruments. (One of his songs that he wrote for he and Tina Cole was used in the Season 12 coffee house ep, with her singing it, though cut from most all prints available.) Grady went on to work extensively in the music industry, for example composing the theme for the Donahue talk show.

There's no doubt the spinoff would have been a sitcom starring Don and Tina & the triplets living in San Francisco; I hope someone has details as to why CBS never pursued it. I've listened to and watched just about every surviving M3S cast member's interviews, and none have ever stated reasons for its not being picked up. Likely none of them know why, although it's possible Grady's inclination to move on to music precluded any involvement on his part.

In her interviews, Tina Cole is inconsistent at times when talking about the circumstances or timing regarding when Grady decided to end his M3S series and/or spinoff involvement. Her memory is not the best -- and unfortunately, she was much more likely than any of the interviewed "Sons" to have had the answers you seek. The other problem is that most interviewers lack familiarity with what was going on regarding Seasons 11 & 12 in order to ask pertinent questions.

One other thing deserving of mention:

Some people on the net have speculated that Don Grady & Tina Cole breaking up in real life was a major factor, if not THE factor, for Don ending his M3S connection when he did. They had a complex relationship (that involved other partners at times): when one wanted to get married, the other was not free, then the reverse, etc. But there was never any animosity that would have precluded their continuing to work together...!

I highly doubt that Don Grady would have turned down a spinoff-series offer or left after Season 11 solely because he and Tina were no longer a couple -- though of course, that may well have factored into his decision.

Here is what "TV Guy" wrote in this forum way back in 2001:

"The last episode of the 11th season was the pilot for a proposed spinoff with Robbie and Katie living with the twins in SF. The spinoff was not picked up, and Don Grady chose not to return to M3S. So in the first episode of the 12th season, it was explained that Robbie had been temporarily transferred to South America, and Katie moved back in with his family while he was away, so they could help her out with the triplets."

By all accounts, the reason Don Grady did not return to M3S for Season 12 is because he decided he wanted instead to concentrate on his music career. He had been involved with music all through the seasons, co-stars have mentioned his practicing being heard off-camera at times and that he was skilled at playing many instruments. (One of his songs that he wrote for he and Tina Cole was used in the Season 12 coffee house ep, with her singing it, though cut from most all prints available.) Grady went on to work extensively in the music industry, for example composing the theme for the Donahue talk show.

There's no doubt the spinoff would have been a sitcom starring Don and Tina & the triplets living in San Francisco; I hope someone has details as to why CBS never pursued it. I've listened to and watched just about every surviving M3S cast member's interviews, and none have ever stated reasons for its not being picked up. Likely none of them know why, although it's possible Grady's inclination to move on to music precluded any involvement on his part.

In her interviews, Tina Cole is inconsistent at times when talking about the circumstances or timing regarding when Grady decided to end his M3S series and/or spinoff involvement. Her memory is not the best -- and unfortunately, she was much more likely than any of the interviewed "Sons" to have had the answers you seek. The other problem is that most interviewers lack familiarity with what was going on regarding Seasons 11 & 12 in order to ask pertinent questions.

Worsening this problem is that -- as with the BBC and ITV across the Pond -- 99% of the paperwork from that era does not exist as there was no remit to keep it.