Teleporters =>
hop on the first, you're on the island - drop one pyramid, use the teleporter back to mainland.
use pyramid in inventory to get on island, pick it up - hop on island teleporter => you're on the 2nd one.
I was just stuck at this point too. I did get to all three islands using the pyramids and I got the key from the third one.
But how do I get to the cave to the south of the teleporters. The map at
http://beyond.divinedivinity.net/ says that you use the teleporters to get to the cave at the southern tip of the section. The cave is blocked by lava.
Where do I use the key I found on the teleporter island?