'My Three Sons' Dawn Lyn's Husband Starts GoFundMe For Mounting Medical Costs

Former child star Dawn Lyn, known for playing Dodie Douglas on "My Three Sons," has had quite the financial nightmare after brain surgery last year, and on the heels of a new medical crisis ... her husband's asking fans to help them with a mountain of expenses.

As we reported, Dawn fell into a coma last October, following brain tumor removal surgery. Her former costar, Stanley Livingston, announced at the time she was fighting for her life. Her husband, John Reese, tells TMZ ... things got worse on Sunday when she fell unconscious and had to be rushed to ER.

John says Dawn was in critical condition, but doctors finally stabilized her, and she remains in ICU.

He says her initial surgery, last year, was covered by insurance, but a problem ensued that created a financial disaster.

Dawn developed bacterial meningitis after being discharged from the hospital ... and after 8 months, she moved to a facility where she continues to receive daily therapy. John says insurance paid upwards of $2M, but they were stuck to cover the remaining balance, which continues to mount.

John says she still needs constant medical care -- well beyond what he can provide personally until he retires. John set up a GoFundMe to help cut the steep costs they're now facing.

He says he has never asked for money before, but they've exhausted almost all of their savings and will have to sell their home and remaining assets if they don't get help.