Chapter 3-1 is the sixth level of Resident Evil 5.
At the beginning of the chapter Chris and Sheva will stop off at a little island. There isn't much here except a gem in the skull mounted to the left, a brown beetle, and the first part of the key (4 pieces all together). So grab the key and any items you find before jumping back on the boat. Go to the Blue with yellow rings area, and pick up the slate map. Feel free to travel anywhere, but make sure you have some hand or flash grenades, because the farthest area up north with a slate contains a duvalia, and a hand grenade or flash grenade will instantly kill it. If you see that little line on the map, go there, because it contains a RPG. Also, that circle in the middle of the map has chickens and some eggs. Another thing, after you pick up a slate don't drive back to where you picked up the map, as this takes too much time. Also, in the area where East slate is, try to be careful in the water, because there are crocodiles in the water. After you have all the slates, you can drive due West towards the flashing symbol. Once docked, head towards the large gate. Insert all four pieces and enter.
You will came across a waterway and a tower with a Longbow Majini. You may proceed without killing it and head to the village. You have three ways to trigger enemies: First, get into an open place with Magnum rounds, second, try to get the first Magnum (S&W M29 Revolver) beside a corpse (It also trigger a trap that there will be poles appearing around and a few Longbow Majinis, and two Giant Majinis, if you cleared the area first, you won't need to worry about this) and third, to turn a wheel to a bridge (which you must use it). One of you must turn the wheel, and the other one opens the door and remove the pole behind the door beside the wheel and let both of you in. Then go toward a flyover cart and go into the cave beside the cart for the Ceremonial Mask (4000g) or get into the cart immediately and end the chapter.