Alright, I'm going to make this clear, THIS ISN'T AN BAD GAME. in short terms I do recommend this game, but this game does have a lot of flaws, and it's mostly because of Activisions greedy tendencies. Because of it, this game got the butt end of the stick when it comes to Activision being stupid. There is a lot of locked content you probably will never touch in multiplayer all because of Loot Boxes and Microtransaction that trick you into spending a lucrative number of hours. (OR be THAT guy and spend money on the game.) I just need to make this clear before I say anything else.
The gameplay is personally one of my favorites out of the series, the movement is really fast, and really smooth. The momentum of the overall game is more faster than any of the earlier games in the series(in my opinion) This game wants you to be on your toes when it comes with practically everything in the game. From Campaign to Multiplayer to Zombies. If you want a slower pace Call of Duty game, this isn't the one.

The weapons you use in this game are really nice to look at. All detailed, all feel unique, and all are (almost) good. Ofcourse this rules apply per game mode. Example: Shieva is absolutely abyssmal in Zombies but the Shieva is fine in single player. I can guarentee you will probably have a favorite gun in this game due to how nice all of the guns feel.
Campaign overall is not good, it's probably one of the worst campaign in the series, which is a shame due to it being marketed as Black Ops Title. The campaign feels FAR from what Black Ops usually is associated with. I personally think it's fine it takes place in the future, but the game is almost completely unrelated to the previous games. Not only that, but the overall story of the game is..REALLY REALLY confusing..Uncessessarily confusing. It's not trying to be philosophical, it's not trying to be smart. It's trying to be clever with important story details but it ends up flopping due to details LITERALLY being scattered across every campaign mission AND THE BLACK OPS 3 COMICS. in a nutshell, To understand BO3 Story, you have to go through an external source which is the BO3 Comic. Why? Because a lot of important things happened off screen. I'll give a good example that I will try to avoid spoilers with.

DYLAN STONE. He is a character that does NOT appear in the BO3 campaign at all, he is only briefly mentioned in a log, but if you read the BO3 Comics you would understand that he is probably one of the most important characters for BO3 story entirely. It's an absolute mess, I wish it was about the Mason family but it isn't, it's a shame too because one of the skins in the game almost looks like Alex Mason.

(going to keep Zombies and Multiplayer Brief)
Zombies is really nice, they really nailed it this game with improving upon the Zombies formula. The maps are all good, unique, and really fun to play. I wouldn't even classify BO3 having a "bad" map, just less popular choices. (Zetsubou isn't a bad map y'all. Don't be so harsh to it.) Zombies Chronicles is easily one of the best things to ever come out of BO3, it makes zombies mode have more content than it already has...but than they added Workshop to Black Ops 3 now the game is just OVERLOADED with content. so there goes what I just said lol

Multiplayer is great, it's one of my favorite out of the series, the jetpacks and the awesome movement system is the reason for it. I know a lot of people dont prefer call of duty having jet packs, but I do think it is important that it exists so it can improve what Advanced Warfare had and made it more fun and accessible to anyone. A lot of the maps are pretty nice to play on, some of them are really freaking unique though. the game feels pretty balanced in terms of guns, perks and killstreaks.

Do you know whats NOT balanced though?
Microtransactions and Lootboxes, I already said what I had to say earlier, but multiplayer REALLY does lack because of it. This is one of my favorite Multiplayer experiences in call of duty ever, but it's freaking ruined by content I know I will NEVER get in my life. Which really turns me off, and a lot of other people. If the game wants you to replay it, it should always feel like a reward replaying it, but for the multiplayers case? It's barely ever feels rewarding. It only feels rewarding for your wallet because now it's emptying a few pounds

In the end Black Ops 3 is a really good game filled with lots of love and content for Black Ops fans, but Activision really f'd it up with it's scummy microtransactions, and the stupid cosmetic bullcrap that awfully plagues this game. I really do like this game a lot, the way they did this game makes me think that this game is definitely going to remembered more than other Call of Duty games, but than microtransactions and the awful campaign is whats probably going to be talked more about than the actual strong points of the game.
Sadly I guess all trains have to go boom some day.