From a comment I made on an earlier post.
It's a rule of thumb I use when putting together meals -- for getting three vegetable side dishes on the table with a minimum of fuss.
I don't always have three vegetable (or fruit) side dishes on the table, especially if my main dishes have plenty of veg in them, but it is nice to shoot for.
"Raw" includes most salads, fresh cut fruit, and also simple things like a bowl of baby carrots.
"Plain" is anything steamed, boiled, or roasted, either truly plain or maybe with a little butter or oil. Roast sweet potatoes, plain microwave-steamed fresh vegetables, or cooked frozen vegetables. Also something like applesauce would count here. The essence of "plain," though, for the purpose I mean (not doing too much work at dinner time) is "simply prepared." I didn't have to stir it or cut it up much or measure anything.
"Fancy" would be anything cooked any other way, or anything that required a recipe. Sauteed broccoli with olive oil and lemon; cream of tomato soup; stuffed squash; marinara sauce. That kind of thing.
"One raw, one plain, one fancy" puts a variety of textures (and usually colors) on the table without my overdoing it in the last minutes before supper.