H.F.
It is great that you are noticing the difference. I know you probably feel horrible about it and know from your statement that you hate the feelings.
Please call your doctor and let him know what is going on. It is probably just all of the changes in hormones. There are little things you can do. Keep a chart/diary of how you are feeling. When you start feeling really angry, right down what you are feeling. Just getting it out can help to alleviate some of the feeling. Also, keep track of what you are eating. During my pregnancy, I had gestational diabetes. I noticed a correlation to my feeling depressed or easily angered for little or no reason to when my sugars were dropping low. You may not be diabetic but with everything else going on and a 3 year old, your sugars may be running low when you are feeling that way.
While it is probably just normal hormones, let your doctor know because it may signify that you may be more prone to postpartum depression with this one as well. If you and your doctor are tracking it, you will be more likely to know if it becomes something more serious that you need to deal with in another way.
I had my 3rd child 7 months ago. The other 2 are 8 years and 3 years old. (The oldest a girl, the younger 2 boys). Some other advice. Try to rest during the day. Break down and let the three year old watch some television that you are not interested in so that you can lie on the couch and close your eyes for a bit. When things become a little overwhelming, call a friend or your mom or someone you trust just to talk. Doesn't matter what you talk about. Sometimes you just need the adult conversation. If things get to feeling to overwhelming at home with your 3 year old, load your daughter into the car and head to a McDonalds or a Mall with a play area. Get yourself a frilly drink and watch her play somewhere that you don't have to clean up when she is done. Sometimes it is little things like this that can make a big difference in your day and your outlook. Remember, it doesn't have to be someplace that will cost you money. The park and the mall play areas are free. If you don't mind shelling out a little money occasionally, the Mall at Robinson and Pittsburgh Mills Mall both have Giggles and Smiles (a pay by the 15 minutes increment play area and babysits your child for you so that you can shop the mall while they play. They are pretty reasonable.). Also, look into a Moms Group like the MOMS Club of Pittsburgh East. (I belong to that one.) Sometimes just having a playdate where your child gets to play with another child and you get to talk to an adult can make a big difference too.
I hope this helps. Good luck with your pregnancy and your 3 year old.