Good New Clues
The "David C Cook/Cook Communications" VBS for 2005 is "Good News Clues" (no longer in print - check current VBS's themes to see if something similiar is available).
Some of those lessons are based on the "lost" parables, as suggested by Cathy W.
If you're looking to build as you go, you might want to break it into 3 parts to go with your 3 programs: why Jesus had to come, who is Jesus and what he did here, and why he had to die.
The other thing is, and this is just my opinion based on our kids ... a full year of detectives and looking for clues is a long time. You may want to use that for one of the 3 programs depending on which it would best lend itself with the stories you'll do. But you know your kids best, so let us know what you decide!