You definitely need some type of insulation. Something that should keep you as warm - or a bit warmer - than what you'd want the sleeping bag at. Most report only being able to go to 65-70*F without something underneath (and laying on a sleeping bag doesn't count as it crushes the insulation).
Depends what you consider three season, but you should at least consider a closed cell foam pad - like a walmart blue pad. Or, if you want lots of comfort and to stretch the temperature lower, a 30 or 40* UQ would be worth considering.
As a note for both, I personally like a 20* UQ and 20-30* quilt or sleeping bag for the rocky mountains in Colorado for 3 season use. You could probably get away with a bit less, but not by much.