These challenges are pretty straight forward and don't really merit a guide but if you're grabbing the game and strapped for time on some here's a quick way to grab three off the bat. At the very beginning, as soon as you're able to control Snake there will be a guard on the watch tower with a searchlight and two guards to the left chatting it up. To the right of the guard in the watchtower is a ledge you can climb up. There's a rat there to shoot. Then jump down climb up the watch tower and hold up the guard, asking him for some info using LB. Once he gives you info shoot him in the head, then look down and headshot the other two guards mentioned before. Now restart checkpoint. One go nets you one rat, one hold up and three headshots so you can rip through those 3 challenges in about 5 mins. Again there are many chances to net these challenges during regular gameplay. This is just an easy way up rip through them.