For those who would like to join but know nothing about the gamemode ill try to explain it a little bit
Arma 3 King Of The Hill is a gamemode where 3 teams try to be the king of the hill, in other words. 3 Teams try to take over a city by fighting against eachother. To capture this city your team needs the most players in the city, if your team has the most players in the city your team will start gaining points. To win your team needs to earn 100 points first.
There is also a leveling and money system in this gamemode. You level up by being in the city does not matter if your team has captured the city or are trying to take it over, you also gain levels / xp by killing enemys, reviving teammates, dropping of teammates in the city and a couple of other methods. If you level up you will most likely unlock something new like a new weapon , vehicle or clothing. The money is needed to buy weapons or vehicles.
There is also perks in this gamemodes, you can have a maximum of 3 perks and you will start with one, the second one will cost you 10k and the third one 100k. There is alot of perks in the game like, medic giving you more bandages and making you heal teamates faster. Engineer so you are able to repair cars. Launcher making you able to buy a rocket launcher.
The rest you have to find out yourself since thats all i really know
PS: i know my spelling is shit.