For this i advise you that you buy the intel pack on EVERY mission you play , except the survive missions, so you can see exactly where all the collectable items are!
If you collect all the star medal from the beginning (from chapter1) then you will have all required star medals 3-4 levels before the end of chapter 3.
Every time you have enough Star Medals available, a flashing sign will be shown every time you are about to start the next mission guiding you to purchase the upgrade for your troops.
Also by buying the intel pack will help you finish the missions faster as you can also see where the enemies are!
Good luck :)