Hi, H.. Well, I think that 3 is a good time for boys and girls to have separate rooms. It sounds like they are all potty-trained, so they need their privacy. It also depends on how well the girls get along. You might try having the girls have their own "slumber party" to see how well they do sleeping in the same room. At the same time, you will also be checking to see how your boy handles sleeping all by himself in his room. I am very, very big on giving kids gradual transitions and trial runs to get them accustomed to changes rather than giving them quick changes that may cause drama and confusion.
Now, were you planning to move the 3-year-old girl into her big sister's room? Watch out for territorial issues. Make sure the big sister is willing to share what used to be completely hers.
Or were you going to move your son out and have the two girls share the room where the twins slept together? That might be easier on the girl twin, but harder on your son and eldest daughter. You might try playing musical rooms for a few weeks and see what works the best for all the children.
Peace,
Syl