Replaced serpentine belt three weeks ago and it has already worn out

What does "worn out" mean? Is is squealing? Is it shredded on one side? What is the symptom or observation that has you concerned?

That is caused by one of the pulleys being turned or tipped in relation to the belt. That rarely occurs to the power steering pump or generator. It commonly occurs to the idler and tensioner pulleys. Sight down from on top and you should see only the part of the belt closest to the top of the engine. (My description is very poor). You should not see the edge of the belt peeking out on the lower pulleys. If you see an area where the edge is visible, meaning off-center, on a lower pulley by as little as 1/16", that pulley or the one just before it is tipped or turned. That 1/16" means the belt is sliding across the pulley as it goes around it, and that is more than enough to set up a horrendous squeal.

You can also observe where the belt is riding on each pulley. The smooth pulleys usually come painted from the factory, and if the belt is riding off-center, you'll see the shiny area to the side of the belt where the paint is worn off where the belt was riding previously in the proper place.