I read somewhere else today (I forget where, but it's undoubtedly true), that the most important thing in minimizing expenses is to control the big three: housing, transportation, and food.
If those are the three expenses that you're splitting with 2 others who don't sound like they're on board at all with cutting expenses then that makes things really tough.
If they're truly not on board with cutting grocery costs, maybe you could just try to phrase it as a non-monetary discussion? "Eat less processed food" - that'd save money. "Cook more from scratch", "eat less red meat", "try cooking some authentic pasta dishes", etc. I don't know what what appeal to them and save money, but you get the idea.
I found that at the beginning with my wife she was resistant to "let's spend less", but if I made specific, positive suggestions then she went along. Once we made enough changes like that for her to realize that "spending less" <> "life sucking", then she was much more receptive.
