If you warped into this world from World 2, you will immediately arrive in the room that contains a door to a Moai head. Collect the head for an extra life. There is little need to explore the rooms to the south unless King Kong is not already at full speed. If he's not, there is a Speed Potions for him to collect in the room to the south. The other potion can only be collected from a different direction.
Instead, from the Moai room, head left and then north through the channel. Beyond the channel, you can collect a Rock Case and another Speed Potion, and attack the boss of this world to collect the key. When you are finished with the boss, be sure to continue traveling south down the left side of the world. You won't want to miss the Artificial Heart before you warp on to World 5.
The boss of World 3 is three fire breathing scorpions. They maneuver back and forth throughout the room, and rush forward to attack Kong. All three have their own hit points, and must be defeated individually. You are actually somewhat safer attacking them from behind, and taking them on from the back to the front, since they rarely back up, and they only breathe fire forward.
Once the bosses are defeated, you'll need to return down the channel and head to the left. You will begin facing some difficult jumps over bottomless crevices. Time your jumps carefully and jump diagonally to bridge the gaps at the narrowest points, to minimize the risk of falling. Once again, remember to travel all the way to the bottom of the left side of this world to collect the Artificial Heart before returning to the warp zone that leads to World 5.