My son is almost 3 months old, breastfed and also has mucusy BMs. Mucus means their intestines are irritated. Does he ever have blood in his poop? It could be a sensitivity or allergy to something you eat. The most common is cow's milk protein. Do you drink milk, eat yogurt or a lot of cheese?
My son started to have mucusy BMs at about 6 or 7 weeks. I didn't think much of it and waited to ask the doctor at his 2 month appointment. I wish I would have called and asked earlier. He said he probably has a sensitivity to cow's milk protein. I cut milk, yogurt, cheese and ice cream as the doctor suggested. After a week it got a little better but there was still mucus. They ran a bunch of tests on his poop and did some blood work. He has blood in his stool (which is not visible at all), low red blood cells (probably from the internal bleeding), but no food allergies. Blood in their stool usually means an allergy to a certain food. I just got the blood test results back today that he does not have an allergy. So, we are still in the process of figuring out what is causing it. For now, I am on a super strict diet with no food products that even contain milk or soy. The doctor said to call in one week if it does not resolve. I hope that this cures it. I really miss cheese and butter. I love to eat them on so many things. I'm really glad I am breastfeeding though. The formula without milk or soy is super expensive. I figured out that it would cost about $250 a month to feed him. The whole family can eat on that.
I would suggest talking to your doctor. Try cutting out milk, yogurt, and cheese until you do. A 3 month old having a BM only every fews days is normal from what I've read. My son goes once a day or every other day.