Doris Roberts' Film Project in Jeopardy ... Cast Keeps Dying

Doris Roberts' death is causing turmoil for a movie she was set to headline ... because she's the third big name attached to the flick who has died.

One of the last movie roles Doris signed on for was the female lead in "Old Soldiers" ... which is still securing its last bit of financing before it starts production.

Problem is ... 2 of the other major roles belonged to Mickey Rooney and James Best ... who died in 2014 and 2015 respectively.

Producer Dave Rotan -- who is adamant on using actual WWII-era celebs for the road trip comedy -- tells us he's still optimistic about the film, but admits his "window of opportunity is quickly closing" on filling out a cast of elderly legends.

Rotan tells us he's not going to recast Doris' role yet -- for fear of jinxing anyone else -- and is still banking on Clifton James (94), Hugh O'Brian (91), and Rance Howard (87) to make it through filming.

Godspeed.