g-fox

You need to have a soldier with the "headhunter" career and look for enemys that have a "red" symbol on their head instead of a white one such as the cqc symbol or the one for handguns,and if it's red,they have a chance of getting S3,now say you capture one with the red color for the "medics" symbol,and you have 10 textbooks(a book you can find in infinity mission with the symbol for what skill it raises)
What you wanna do is save your game and then use the medic text book on the soldier you have until it get's to S3,now if it didn't get S3 and stopped before hand,go back to settings in the menu and go back to the main menu,then load your save and try again,i don't know if reloading your save to try again would work as I haven't tried it yet but it might