You started your thread indicating that you were wondering whether you were reacting selfishly to the situation, so deep inside, you do know that your attitude is not right, but it is just harder when people tell you so.
Once again, problems are already starting raising serious alarm bells because once again comes another situation that has been massively rushed. Within 2 years, you've met someone, had him become a father to your DD, had another kid within months, got married and are taking on a pre-teenager who clearly has some issues if he felt so strongly about moving out of his mum's house. OF COURSE you are going to face issues and difficulties.
What is coming across as is that you had it planned in your head that you would build this perfect family with this man, someone to replace your DD's dad, have another baby with, move in together and play happy family with the other child in the picture but hopefully not impacting on your plans. Now you are finally faced with the reality that this child who means little to you, means as much to your OH as your two children.
On this basis, you do need to put yourself in your OH shoes and realise that it is very wrong that you consider normal that he should so quickly become your child's dad, but think that you shouldn't have to accept his child full-time. That child has as much a place on your family than your two children, however inconvenient it is to you.
You also need to look at it from a 'normal' family dynamic. If you had three children, a boy of 11, a 3yo daughter and a newborn baby girl, OF COURSE the 3yo would be expected to share with the baby once it sleeps through the night. Your statement that she never had a room for herself and your seemingly attitude that she deserves to have one at that age is irrational. At that age, they normally don't like being on their own anyway.
All this is not to beat you up. You are right that you didn't expect to have to share your life with this boy, so this is now coming as a big shock, require a massive adjustment at a time you will feel most vulnerable with a young baby, so you do deserve some empathy. However, you also need to try to understand it from your OH's point of view. He is now asking you to take on his son as you have expected him to take on your daughter. As a matter of fact, less so since you are not expected to act as his mum. It might not be what you signed up for, but when you took your OH, you also took on the possibility that this could happen.
You now need to work together to make sure that it doesn't all fall apart. It's great that you seem to be able to communicate well as this is the most important part. It sounds like your OH is showing understanding of your feelings and be prepared to do something about it, it's now your turn to do the same.
So yes, your 3yo should share a room with her sister. Yes, you should be the one dropping your DD to school whilst your OH takes his son and if this mean that your baby needs to adjust, then that's what needs to happen like millions of babies with older siblings have to do. Don't start resenting your DS. I'm sure he didn't pick to feel that he can't live with his mother any longer, and take on a new SM and two kids by moving with his dad. Compassion is really essential all around. Good luck.