3 month old eating schedule

I'm getting sleep and scheduling advice from a friend who has 4 babies and swears by her technique. Anyway, she recommends that I breastfeed my daughter every 3 hours during the day. Like 9am, 12pm, 3pm, 6pm, then 9pm and bed. Then do whatever during the night and eventually she will start going longer stretches. Is every 3 hours during the day reasonable? I feel like I would feed her all day long so going 3 hours seems a little rough, but she's not screaming in hunger, but I do have to distract her more since she is used to being on the boob every 2-2 1/2 hours or more often. My daughter is slmost 15 weeks and I'd say 13-14 lbs. Also, do I need to worry about my supply, since I won't be breastfeeding as often, or will she just eat more when she does eat?