3 year old always crying

So, a little backstory:

I have 3 girls, 7, 4 (5 in Oct) and 3 (4 in Sept) and I am currently 36 weeks pregnant with our first boy. My youngest (3) is really starting to stress me out, to the point that I’m not sure how to cope with her. When she was a baby, we found out at 4 months old she had colic; she cried ALL THE TIME and we had a lot of sleepless nights, and months of trying to figure out way’s to soothe her. A little after her 1st birthday, she calmed down a lot, and was actually a good child, accept she liked to play by herself a lot and wasn’t very sociable with her cousins or siblings, which we just thought she got that from her daddy. But now, that she’s a little older, she has become a lot meaner, like aggressively yelling at us, slamming doors, and has the worst temper tantrums when you tell her no or ask her to do something; it’s like a glimpse into her future teen years. She is also very OCD and will not eat food that breaks, doesn’t like to be dirty, the list goes on and on. She cries ALL the time, about EVERYTHING! The only time she’s content is when she’s by herself or when her middle sister plays with her in her way. Not sure if this is the terrible 3s, being slightly spoiled by her daddy, or if it’s something more I’m missing, but I need help on how to ocpe with it, especially with a NB and 2 other kids to care for. She honestly takes up a lot of my time throughout the day, and I can’t wait until she’s in bed so I can finally be alone, which hurts my heart to say.