or cod do the snack drawer thing
put in non-perishable non-messy things (or make strict rules about where they are to be eaten) and let them help themselves in the evening
I do understand about wanting to stop messing about with food after a certain time - SAHM thing can be so DOMESTIC and you end up making food, shopping for food, clearing up food all day long - with 3 I'd imagine it seems interminable
I think fruit & cheese sounds good but you would have to prepare toast or veg wouldn't you? let them have things they can get for themselves (oatcakes? bagels? nuts?)