What size opening do I need in CB construction for a 3'-0 entrance door?

If you are using a 36" door you'll need to allow for the door frame and shimming. 1" to 1 1/4" should be sufficient on both sides. Shim under the hinges first until plum. Sim on the lock side top, bottom and lockset until plum. I am getting lazy and tend to go with pre-hung doors.

Rough Opening Is 38 1/2 For The Width. Its Always 2 And A Half Inches Bigger That The Door, Some Will Be Just 2" But It All Depends On The Studs And If There Plum Or Not. If Not Youll Want That Extra 1/2"
Hope This Helps , Nichols Trim And Carpentry