Simply complete the three base game campaigns on medium. Shouldn't be too much of a problem. You can also get this by beating the game on a higher difficulty, so doing a hardcore run through will net you all three difficulty achievements at once.
Edit: if the achievement doesn't unlock when you beat the last campaign, try loading your last save of each campaign and watching the epilogue videos again until it pops. Popped for me after watching the Under No Flag epilogue again.