It sounds like you ready to stop and your daughter is not. I would not allow her to scream, not because you fear that someone will complain about it but you need to do this with love. Breastfeeding a child is an incredibly bonded relationship. To her in her child like mind, she would receive that in such a negative way that it could affect your relationship with her. She trusts you! She trusts that you will always take care of her needs. Breaking that trust would have devastating results.
She will not potty train. Why? Do you work that you cannot consistently train her during the day? Are you enabling her by still using diapers? Have you tried potty training her but then stopped at some point? Once you begin potty training you MUST be consistent. You cannot expect to be fully potty trained without consistency. First thing in the morning, put her on the toilet. DO NOT put a diaper on her during the day. Do not even put underpants or pull ups on her. Instead allow her to run around the house naked. Yes, naked. If you put pull ups on her or pants or underpants, she will use them to potty in. Trust me, this is not a good thing. Naked is best. She will know without any doubts that she has to use the bathroom and even if she doesn't tell you, put her on the toilet every hour and IF she refuses to go, make her sit there until she does go. After she goes, reward her with something that will only be given to her when she uses the toilet. I used Gerber gummy snacks for my children and they only got ONE snack, not to be confused with one bag as there are several pieces to one bag. And the idea is for her to be rewarded (not bribed) for doing a good thing. You might have to resort to pullups at night until she is able to stay dry during the night. The key to night time training to put her on the toilet before bath time and then again just before bed and NO drinks after 8 P.M. and you really need to stick to this no matter how much she yells. She's a child and you are the parent. Make this known to her by not giving in. If she is not staying dry during the night you may have to cut her no drink rule to 7:30 P.M. At 3, she is capable to go to the bathroom all by herself, sit on the toilet, use the bathroom by herself, wipe (if she went pee) and be able to call you for help with bowel movement wiping.
Sleeping in her own bed. Why is she not sleeping in her own bed? Where is she sleeping? Have you been co-sleeping since birth? If this is the case, you may find it challenging to get her to own room/bed but it does need to be done. Designate her space for sleeping today! Tell her she will be sleeping HERE and not in your bed anymore. Use a night light if she is afraid to be alone in the dark. Let her cuddle with a favorite toy. Read her a story before bedtime and then it's lights out. Maybe she would prefer to sleep with her bedroom door open. Ask her if she'd like it to be open for sleeping but if she comes back to your room, take her immediately back to her room. Your husband must help. This could be exhausting for you. The key, again, is to be consistent.
She is underweight? Is this what the doctor has said because if so, I'm really surprised the doctor would not have intervened by offering suggestions to help her gain weight. What does she eat? How often? How often is she nursing? These are things your doctor needs to know. You can give her cow's milk and add those breakfast powders to her milk. I believe one brand is Carnation. Pancakes, will she eat pancakes? Frensh toast? Waffles, oatmeal with raisins, brown sugar, or cinnamon? For lunch, will she eat yogurt? Cheese? Any kind of sandwich? For dinner, do you offer the same things you make for dinner to your daughter? Snacks can be cut up fruit, maybe with yogurt drizzled over the top. Crackers with peanut butter and jelly, tortillas with a slice of cheese and a slice of turkey or ham rolled up. Egg salad sandwiches...slice of bread with jelly...mini bagels with cream cheese. She should be able to eat everything you eat. Don't be afraid to try new things. A child should taste everything. I find that a lot of parents are afraid to try things because they falsely assume their child won't eat it. My 3 year old LOVES sushi, so try different things and if need be, write down what was liked and not liked. Spaghettios are good, too. Soups are good too. Good luck to you. You have a challenge on your hands but you CAN do it. : )