Personally, I think it is a little unfair to have non-USA umpires ONLY work 3B. Can you imagine getting the notice that you will be working the World Baseball Classic....BUT you're only allowed to work 3B.
Don't get me wrong, I think the professional umpires in the US, both MiLB and MLB, are the best in the world - largely due to the training we have here in the states. However, if you are good enough to be invited to work an international competition, than you should be able to work all positions - not just 3B.
The WBC can't have it both ways - they either have all of the umpires from the MLB/MiLB or diversify the group and have a certain number from each country and let them work ALL positions. Heck, they won't even let those 3B umpires carry the stop watch for commercial brakes - what they aren't qualified to tell time either.
I think you guys get my point.