Dawn Lynn — the former child star of TV’s “My Three Sons” — is fighting for her life after undergoing brain surgery and falling into a coma … according to her former co-stars.

News of Lynn’s condition first surfaced on social media via posts from people like Stanley Livingston and Tina Cole … who starred opposite her back in the 60s and 70s when their sitcom was on the air.

Livingston wrote: “Just found out my TV little sister, DOWLYN (Dodie) is in a coma and fighting for her life. She recently underwent surgery to remove a brain tumor. Please pray for her speedy and complete recovery. I took Don to lunch a couple of months ago. We had a great time reminiscing about our MTS days. Don is one of the nicest people I have ever known. Thoughts and prayers also go out to her husband John at this time. This is devastating news!” Stanley played Dawn’s TV brother, “Chip” Douglas.

Cole also addressed this, saying… “My heartfelt prayers for my sweet sister of ‘My Three Sons’ Dawn Lynn (Dodie) and her husband John. Don is fighting for his life after brain surgery.”

She adds: “I am in touch with John and will keep you posted. Please keep her in your prayers.”

Lynn’s real-life blood brother, Leif Garrett tells TMZ … “My sister, Dawn, never wanted anyone to know about the surgery. As this has leaked out on social media, I would really appreciate it if everyone understood that these have already been difficult times, having lost both of our parents and a cousin recently.”

Leif also says, “Please keep my sister in your thoughts and prayers and respect our family’s privacy. Thank you.”