I have a boy aged 9 and a girl aged 7 and a boy aged 2.. All in one bedroom is this right

The girl really needs her own room.
There really is no law in most places (unless you are on sect 8)
But a girl needs her own space and privacy.
Also as they get older you really have no idea what is going on behind closed doors while you are sleeping.

Its not illegal in most places. Your local children's services agency might have some guidelines on this, but very few places legislate it. If they did, there would be a lot more homeless people.

But, its not the healthiest situation. It would be better for the girl to have her own space, especially as she reaches puberty.