F.D.
I have a 3 month old too and was having the same problem until I really did the Eat activity sleep and you (EASY) routine (not schedule but routine) recommended in the baby whisperer.
I found that I was misreading her tired cues for hungry cues and had gotten her in the habit of nursing whenever she felt tired. So it was more like nurse, play, activity, nurse, sleep, etc.
You might want to try to get her to sleep the next time you think she's hungry. Now my baby sleeps for about an hour, nurses for 30 minutes, plays for 40 minutes or so and then sleeps again. It's been much easier on me and she seems happier too. Good luck!