It would help to know a bit more about the conditions you'll be using it in: are you in the western mountains, the Pacific northwest, Sierras, Tennesee, Michigan? What are the lowest nighttime temperatures you expect to encounter? (45 degree days, in some seasons, here in Ohio can easily mean 20 degree nights.)

My own 3-season, all-purpose bag was, for many years, a Western Mountaineering Megalite. A couple of years ago, I replaced it with a 20 degree Alpinlite and a 40 degree Mitylite, which better fit the temperature range I hiked in. (The Megalite was far too warm for summer, and not warm enough in January.)

If you're strictly a mild weather camper, and your nighttime lows will not go below 35 or so, you might be OK with a hoodless semi-rectangular bag like the Western Mountaineering Mitylite; it can be used as a regular bag, or unzipped and used as a quilt. In temps below 40, you'll probably have a hooded jacket of some kind (mine's a WM Flash), and you can wear it to bed with the hood pulled up.