Was very thirsty the other day, and took eight (count 'em, eight) hard swallows of water before I could yank the bottle from my own lips. Interesting how generous each character in this drama was to hand over the canteen without a fight.
I think that, in real life, they'd have been given fixed portions. Too easy to cheat - with three bigger gulps, or with extra kept in the mouth after the third swallow to be swallowed later, or just an extra swallow or two "accidently" consumed after #3. Not to mention cheating by the enemy personnel. I guess the "three swallows" made for better dramatic effect, and/or was easier.
They were all in the military, accustomed to taking orders, and supervised. They weren't drinking to slake their thirst completely, but drinking their ration.