Generally, i would just look at it from a training perspective. Which two are you willing to commit 14 weeks a season to to improving as much as possible? Anyone who has potential that doesn't fit that equation should be an immediate TL listing, as you can fill the 3rd training spot with someone that is of less quality.
I'm not sure who you like the best, but that should define who you train. If you can't decide on the top two, decide on your favorite and decide who would best fit into the training regimen you put in for the first guy.