T.B.
FIrst your baby is probably on a growth spurt. Second, breastfed baby's fall asleep at the breast...normal, wonderful, relaxing and an all around great way to go! 3 weeks is still too early to introduce a pacifier. You are still building your milk supply and having him fall asleep while nursing is a great way to build it. Your baby does not need an artificial nipple...all he needs is you!
That said, you can look for cues that he is not longer nutritive sucking. He is just holding the nipple in his mouth, does not suck rhythmically, is asleep. It is okay to remove him gently from your breast and put him to bed. IF he is really asleep this will not disturb him as all. If he is still hungry he will let you know! LOL
You are doing great giving him exactly what he needs right now..keep it up mama!
HUGS T.