Soften cream cheese for a few seconds in microwave if necessary. Heat cream, butter, and cream cheese in a small pot, just until the butter and cream cheese melt into the cream. (Stir frequently to avoid burning.).
Remove the pot from the stove, and stir in the 3 cups of powdered sugar. You will have a goopy mess. Beat it with your electric mixer, until it changes color slightly (3-4 minutes.)
Spoon, paint, or spread it on your cooled cookies. It will still be pretty thin and goopy, but it should spread in a nice, even coat. (I used the back of my spoon to spread it.)
Let the icing 'set' for 1 1/2 - 2 hours, until it hardens. It won't harden completely, but it will form a nice soft frosting layer that squishes only slightly when pressed, similar to a slice of cheese.
I modified a regular icing recipe to make this, and was very disappointed with the results at first, but after I left the cookies on the counter for two hours, I was glad I saved them. The icing is perfect for transporting, and has a very good flavor, just right for carrot cake cookies.