At 3 weeks, you hardly have to start weaning them...it's much too early. What you do, is once they hit around 4.5 - 6 weeks, you leave a very shallow dish or lid of a mix of KMR and a bit of canned wet kitten food out for them to investigate, and hopefully, they'll at least try it. Do this once every 3-4 hours, and gradually each day, add a bit more warm mushed up canned wet kitten food, and reduce the amount of KMR. You can even leave out a bit of dry kitten kibble in a lid (soaked in a bit of warm water to soften it), but don't leave this laying around for more than an hour or two at a time due to bacteria.
Good luck...weaning is surprisingly easy, but it's still too soon at 3 weeks for them to not be nursing. In fact, many litters will still be nursing at 5 weeks...it's not uncommon.