Feed him crickets no bigger than the space between his eyes and let him eat as many as he wants withing a 10-15 minute time span. Some days he may eat more, others less, depending on the dragon. Mine always eats less when he is starting to shed but once the old skin is gone, he eats like a pig.
Not sure how far away from home your work is but you could always come back home during your lunch over and feed him then. If you can't then two feedings should be alright. Don't forget to mist him and leave a small cap of water to drink (if he does that is) so he won't get dehydrated.
3 weeks seems pretty young for a beardie to be sold...mine was three months old when I got him, and about 8 inches long. As long as yours is 6 inches, 2 feedings a day should be ok, as long as he has greens for the time between feedings.