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.