<P> The following year in the tenth season , 1969 - 1970 , Steve remarries , taking widowed teacher Barbara Harper ( Beverly Garland ) as his wife ; she brings with her a 5 - year - old daughter , Dorothy `` Dodie '' ( Dawn Lyn ) , so Steven now had a stepdaughter whom he also subsequently adopts . Also , the last year - and - a-half of the series feature fewer appearances of both Don Grady and Stanley Livingston . Grady 's character was written out of the show at the end of the 11th season , which allowed for his wife Katie and their triplet sons to remain within the Douglas household the following season ( as a structural engineer Robbie was working on a bridge construction in Peru ) . Chip and his teen wife Polly ( Ronne Troup ) ( who eloped after Polly 's disciplinarian father refused to sanction the marriage ) move into their own apartment . </P>
<P> The following year in the tenth season , 1969 - 1970 , Steve remarries , taking widowed teacher Barbara Harper ( Beverly Garland ) as his wife ; she brings with her a 5 - year - old daughter , Dorothy `` Dodie '' ( Dawn Lyn ) , so Steven now had a stepdaughter whom he also subsequently adopts . Also , the last year - and - a-half of the series feature fewer appearances of both Don Grady and Stanley Livingston . Grady 's character was written out of the show at the end of the 11th season , which allowed for his wife Katie and their triplet sons to remain within the Douglas household the following season ( as a structural engineer Robbie was working on a bridge construction in Peru ) . Chip and his teen wife Polly ( Ronne Troup ) ( who eloped after Polly 's disciplinarian father refused to sanction the marriage ) move into their own apartment . </P>
who played the mother on my three sons
who played the mother on my three sons
Beverly Garland
Beverly Garland