Should I keep this secret?

My senior year in high school I was in love with my boyfriend but we ultimately broke up because my best friend and him couldn't get along..now 3 years later we have reconnected and gotten back together all while he is over in Iraq, but a year ago not thinking that he and I would EVER get back together and making a HUGE mistake in the process I slept with his best friend...now I'm torn on whether not I should tell him and if I do how do I break the news to him?(Any good advice would be appreciated.)