I breastfed for 14 months and mine look just fine. Just make sure you feed your baby evenly on each side whether its both in one feeding or switching between feedings. I heard about on girl only fed on one side because thats what was easier for her son and didnt want to try forcing him to take the other and ended up with an A on one side and a C on the other. Also if your worried about sagging pregnancy is the main culprit because of all the stretching of the skin from them growing.
I breastfed my son for 2 full years. My breasts certainly aren't quite as full and perky as they used to be but they're still bigger than they were before he was born and with support they look pretty damn good.
Why would you stop nursing after 3 months? The benefits of breast milk last much longer than that - and the difference in price between breast milk and formula... well, you do the math. Pretty amazing how much something so tiny can drink in a day.