The difference is if you choose 3 it will pick 3 of the same ships with the same loadouts
In the pic below all 3 ships will be the same class and the same cargo - Freight\freight containers
If you add one at a time you can alter the cargo carried and also the the crew rating
Plus you get a better mix of shipping
Hope that helps
Damn didnt see the bit about escorts
Do'h
Same difference
If you choose 3 it will pick all 3 escorts the same class and experience