T. I am so there with you. This is my third pregnancy and this is the first time I feel like I am being mean - and only mean to my three year old twins, my 8 mo luckily gets the protection of being just a baby.
My main problem is that I am much much more unable to handle stress, especially the kind where we have to get out the door, where they are procrastinting doing what I ask, where I am exhausted and they wont nap too.
One thing that is helping more is that I am being upfront about my feelings with them instead of just snapping at them when they push push push. I will say to them that they need to stay quiet in thier room because mommy is tired and needs to sleep (we have a gate so they have to stay). Since they often insist they need no sleep even if they do, we discuss how mommy will feel if I don't get a nap(cranky) and how mommy will feel if I do get a nap (Happy and ready to play with them). I then remind them that for me to sleep they need to play with thier toys/dolls/books in bed. It works about 50% of the time. Three year olds are beginning to understand empathy and it helps for us to show them we have feelings to. You want to use I statements like. "When you do not get shoes on, I get frustrated and angry because I want us to get to the library on time to enjoy the program. Do you want to miss some of our fun at the library?" You statements like "When you do not get shoes on you make me frustrated and angry" don't work as well and encourage arguement.
I will also model self regulating behaviour for them. I tell them straight up, "Mommy is very frustrated and I need a time out. I am going to walk up to my room and take a few minutes. You need to do X Y and Z while I am gone if you want me to play with you when I am done".
My friend aslo got me a chart with faces on it and I am going to work with them to help them understand and name feelings. I also have a thermometer and we are going to work on some words to use together if we are getting into the red zone so they can learn to tell me how they are feeling as well. I am sure you can find one on the internet.
OH and preschool is great IF you can afford it, I wouldn't go for all the time though, just a few hours a week can make an amazing differnce in your patience level and can help you to bond with baby after he comes.