You need to do research into how the MB vendor wires up the PLX bridge chip as well as the lane switches... think of it as railroad tracks.
You could theoretically wire up x16/x16/x16, but think about the number of lanes, and how those are multiplied by the PLX chip... in reality, any vendor running a PLX chip would want to maximize the usage of the lanes, and that usually ends up in a x16/x8/x8. or a x8/x8/x8/x8 configuration.
But x16/x16 I'm not sure. Have not heard of anyone doing that because it's a waste of a PLX chip for very, VERY minimal returns. Keep in mind a PLX chip costs quite a lot (compared to other aspects of the board)...
For more info I suggest you read this:
http://www.anandtech.com/show/6170/four-multigpu-z77-boards-from-280350-plx-pex-8747-featuring-gigabyte-asrock-ecs-and-evgaEdited by mav1178 - 5/30/13 at 9:43am