Since Harry can paint the room in 3 hours he can paint the of the room in 1 hour.
Using the same argument for the other's we get
Annie of the room in 1 hour
Mitch of the room in 1 hour
Now we can add the rates that each of them work
Simplifying we get
so it will take them One and one thirds hours or 1 hour and 20 min.
I hope this helps.