I feel beby veileds are extremely hardy, even when young. The biggest problem is feeding. When they're small, they lose weight so quickly, that the 24 hour period of shipping, coupled with the potential for an acclimation process, might rob them of vital energy. That's why I never sell veileds until they're eating at least 1/4 inch crickets. For me, that's usually 2 months old.
Others have much larger animals at 2 months old, but I do it for a reason. When grown at a faster ratem, they are much more prone to MBD. Doesn't mean they're gonna get it, not at all. It just means they're more suseptable to it. They end up taking about a year to reach full adult size, as opposed to 6-9 months for faster grown babies.
Regardless of age, I believe it's the size of the animals that matters most when it comes to selling. I've never heard from a customer that their chameleon was sick or dying - and I sell at 2 months.
I agree with Valerie - pardalis are a different story. Actually, everything else is a different story than veileds! I probably wouldnt' buy another chameleon unless it was 3 months or older, unless I was picking it up from a friend, and sparing it a UPS trip...
I got some CB melleri at 2-3 months, and they were fine. they might be the only other chameleon that's sturdy enough at 2 months that I'D consider shipping. Considering the rarity of them in captivity, I don't think we'll see many people shipping them until they're ROCK solid though!
Now for the question:
What is the 3 week old eating? If it's pinheads and fruit flies... I'd be anxious. If it's 1/4 inch crickets, it should be o.k.