Call of Duty: Black Ops III includes a new difficulty level called Realistic. In Realistic mode, your character won't survive one single shot, which is rather... realistic, if you think about it. Anyway, it's a pretty tough challenge for your to embark on, so you might need a few tips in order to face it with the best amount of possibilities possible.
First, Realistic won't always kill you at the first shot as the developer Treyarch "promised" while promoting the new Call of Duty. In a few chances, especially when facing particularly strong enemies, you will be provided with an extra shot. There's a good amount of chances this is a bug or glitchy thing Treyarch will fix soon, but it happens with immortals or in the Zombie modes with Demon Within.
By the way, this doesn't mean at all you won't die. Call of Duty: Black Ops III can be a very frustrating experience at the Realistic difficulty level, especially considering you are facing a game which has been thought about as a cooperative and competitive experience. Locations are pretty big and it's often a matter of finding the right spot where you can easily shoot and get rid of your enemies from.
A few tips will save your life. You need to always stay behind a cover and remain calm, relying a lot on your AI companions which reveal particularly useful here. Active Camo will also play a huge part in your playthrough (use your cores as soon as you can and don't move until you don't have them recharged!) and get the trophy system ability that pushes away explosives up to the rockets pushing away ability. Rely a lot on your DMI view as it will help you have a better vision on what happens in the battlefield, calculating where grenades are going to drop and where your toughest enemies are.
Ultimately, it is highly recommended not to play Realistic at the first playthrough, as you really need to have a good memory of the maps you are going to play in and an overall understanding of what's happening there. You will notice the game gets really harder to beat in the second half and in particular has four times where you will be stressed up to your very limits:
As for the zombie, remember they won't move if you are invisible. In the core battle, beware of robots: the most hard to beat will both run and shoot at you, so you need to be really careful about their movements while you're under cover. Camo will help in the other areas.