I suppose you are making a minigame plugin and you want to get the player(s) that have the biggest number of kills. right?
Try looking into this. http://www.mkyong.com/java/how-to-sort-a-map-in-java/ It's not exactly what you want to do but gives the general idea of what you would want to accomplish.
Very Information. Such helpful. Many context. Seriously, if you need help, give Information on everything connected to your problem. You said you save all the Kills and their Names/UUIDs (dare you save the Players itself). To sort the whole HashMap, take a look into sorting systems. Choose the one you prefer, I recommend taking one which directly sorts the 3 highest Values as first (e.g. BubbleSort). I recommend stopping the sorting after you got the 3 first ones. The rest just takes performance.