My baby is three weeks old and has baby acne. Is there anything available to treat this?
No, but you don't have to.
Before birth, maternal hormones (as well as oxygen, nutrients, and other important molecules) cross the placenta and access the baby's blood stream. As a result, newborns have higher levels of certain hormones during the eight weeks or so it takes for those molecules to degrade. One consequence is a temporary skin condition called infant acne, that looks and acts like a mild case of teenage acne (the latter is also caused, in part, by a surge in hormones).
After about eight weeks, these maternal hormones are essentially gone and not replaced by the baby's system. So your child's skin condition is transient and will resolve on it's own with no special treatment.