Well not everyone had the same answers as me (see comments in post below)... but some were close. To solve this riddle, we need to start with what we know about the boys.
Luke hoards money and is frequently making deals to get more money. He thinks he should be paid for getting out of bed in the morning. He will go to the store to buy a $30 item, and will leverage parental equity to pay for the sales taxes. He also is giving and has been known to give money to parents, grandparents, and brothers. B-School for Luke.
Caleb... Legos... Legos... Legos... Never concerned too much about money. Likes to read intricate Lego procedures in his spare time. A little eccentric and not concerned about what he wears. Focused on the task at hand (when it's something he wants to do). Strong in science in school - to heck with everything else. Likes to help people. Med School for Caleb.
Seth... Could do anything, I think, but there is a sense of justice and logic to him. Having two younger brothers who tend to cause him some issues on occasion, Seth has to argue his position often, and is usually pretty calm and level-headed. Often is able to identify how brothers have wronged him in some way, and convincingly proclaims his innocence in the matter. Law School for Seth.
Of course, I would not be disappointed if I have all this wrong. All of these are tough fields of study followed by time-consuming careers. I just want each of them discover God's will on their own, and to make the best of their lives by living that out.
So where's the engineer and jewelry designer in the family? I guess that is Dad and Mom! And I'm still thinking about opening that hot dog stand in the shallow waters of the Keys in Florida.