Please don't listen to your family and hit your child. You are her role model and she is only going to mirror whatever you do to her. You are an adult. You don't for ANY REASON need to be hitting a 3 year old. If you show her that hitting is an acceptable way to deal with YOUR anger and frustration, then she will just think it is acceptable for her to do the same. You're her mom and she looks up to you. Considering that your family has told you to hit your little one, maybe they are where she is seeing that behavior.
Also, you said that you "yell no" at her when she is misbehaving. Same thing here. She is only going ot mirror what you do. If you are yelling at her, why wouldn't she yell at you? It is a normal and healthy part of toddler hood for her to start trying out her authority by telling you 'no'. It is part of them learning independence.
Something that has worked well with me and kiddos around that age, has been to phrase my questions / demands / directions differently. Instead of "Get in the car now please" try "would you like me to help you get in the car, or would you like to do it yourself?". Try to give them less yes or no questions and more choices. This way, they can still feel like they are in control of the situation by choosing what they want to do, without you asking a yes or no question.
Washing her mouth out with soap is definitely not okay for her, and could be damaging to her body. I nanny for toddlers, and when they decide to get sassy or rude, I tell them if they would like to talk like that, they need to go somewhere else. If it's summer and you can let them go outside, do it. Otherwise IGNORE it! And tell her you would understand she is frustrated, but can't talk to her until she can speak to you in the same - CALM - tone you are using with her. She is most likely talking this way to you because she has found that it is a great and easy way to get your attention in the past, and gets a reaction from you.
You might also want to think about how she may feel threatened by the fact that you are having another baby. Alot of kids sor tof freak out when they think they may loose attention or their spot as the 'baby'. I'm sure alot of talk and energy has been going into the new kiddo on the way, and this may just be her way of testing to see that you still really care about her. Or getting back some of the attention she feels she's losing to the pregnancy. Kids will take attention in any form it comes in , even if negative attention.
Anyhow........ it sounds like maybe you need to take a few breaths and treat her with a little more respect. She will eventually follow your lead. Make sure you are giving her a TON of praise when she does even little things that you DO like. But if you choose to hit and punish her negativly , it's my opinion that she is going to do the exact same to you.
if you have the time, or you a reader I would reccomend a GREAT and easy to read book that has really helped me - calle PARENTING WITH LOVE AND LOGIC.