In a large mixing bowl, cream together butter, shortening, and cream cheese.
Slowly add in powdered sugar and continue beating until well combined and creamy.
Add vanilla and 1 tablespoon of milk and beat again until combined. If mixture is still too thick for your liking, add an additional tablespoon of milk and beat again until you reach you desired consistency.
This is a large batch, which can frost a full size cake or up to 48 cupcakes. You can split the recipe in half for a smaller batch.