The oldest of the three - original Korean version was released in 2004 - is War Rock. That has downside - graphics are visibly dated and may be not for your liking if that is what you seek first. On the opposite, if you have a PC that is not exactly a high-end gaming rig, you can still enjoy this game, especially, if the primary function for your computer isn't gaming and you just want to jump in and have a quick match.
It is typical for the games of this genre to have some resemblance with one or another "classical" multiplayer shooters. In the case of War Rock, it is most certainly Battlefield 1942. That can be seen both in character classes - medic, heavy trooper, engineer, sniper, assault, and in the fact that there is a game mode called Battle Group with huge maps and useable vehicles. There are also couple of modes that offer more classical gameplay for smaller number of people - Close Quarter Combat and Urban Ops.
The gameplay feels a bit arcade-like. It's not a bad thing, and I actually enjoyed it. While running around and shooting, you can roll, fall prone and sprint, which gives some tactical options to face your opponents on your terms or avoid them. All in all, as I have said, everyone that has played Battlefield 1942, will feel at home here.
Game utilises a standard scheme for the genre - you earn in-game cash and expierience, which allows you to rent better gear for your soldier. Yes, rent, as it is quite typical in this type of games to be able to buy stuff permanently only with real money. War Rock doesn't give you such option, but paying has its benefits like faster leveling and discount in the weapons store, as well as special items, crosshairs and many more.
NOTE: While I have no such bad experience myself, I have heard lots of complaints about cheaters in this game using hacks.
So, overall, what we have here is a bit simple and dated game, which is quite fun, though, and is not too heavy on the system resources.