I think i accidentally drowned my rat what do i need to do to not make this mistake again?

I have 3 females. Well i guess i should say had. Okay so i filled up the bathtub with about 3 inches of lukewarm water and decided to bathe the 3 of them at once. i used a small pet shampoo called Squeaky Clean and i took one out to put her back in their cage and came back and 1 was dead keeled over for no apparent reason other than drowning! I've bathed them once before and never had a problem. I guess my question is what do i do so i know better for next time? God i feel so ******* horrible right now!