While the three sons were always central to the storyline, several major changes took place by the late 1960s. In the spring of 1967, the ratings for the series began to sag and My Three Sons finished its seventh season in 31st place in the Nielsen ratings. It was decided that the 1967-1968 season would bring the program not only a new time slot, but also new storylines to spice up the ratings. In the fall of 1967, CBS moved My Three Sons to Saturday night at 8: 30 pm. In the season-premiere episode, "Moving Day", the Douglas family and Uncle Charley relocate from the fictional town of Bryant Park in the Midwest to Los Angeles. Robbie (Don Grady) marries his classmate / girlfriend, Katie Miller (Tina Cole). Tina Cole, in fact, had appeared in different roles on three previous episodes of My Three Sons: "House For Sale" from the fourth season (February 13, 1964), "The Coffee House Set" from the fifth season (November 19, 1964), and "Robbie and the Little Stranger" from the sixth season (February 17, 1966). At the end of the 1967-1968 season, the ratings had improved from the previous year with the series placing at 24th in the Nielsens. The following season, the newlyweds discover that Katie is pregnant, and she gives birth to triplets named Robert, Steven, and Charles. Although originally played by sets of uncredited twins, these babies were played uncredited by Guy, Gunnar, and Garth Swanson. The most familiar triplets in the show's last two seasons are played by Michael, Daniel, and Joseph Todd.

Based on this passage, who played robbie douglas wife on my three sons? Let's give stream of consciousness first:
The relevant information to answer the above question is: Robbie (Don Grady) marries his classmate / girlfriend, Katie Miller (Tina Cole).
So, the final answer is Tina Cole.