no, peanut butter itself will not cause a gain in weight. you gain weight by eating more calories over the course of the day then you burn off. natural peanut butter is good food because it contains a good amount of unsaturated fats which are important for the body.
personally, i believe in meals every 3 hours over 3 meals a day. there is good proof out there on how smaller meals more often during the day has a better effect on body composition.