Ok, here is the REST doc on the "all:3" syntax...
Description: The layers to perform the identify operation on. There are three ways to specify which layers to identify on:
* top: Only the top-most layer at the specified location.
* visible: All visible layers at the specified location.
* all: All layers at the specified location.
Example: layers=all
Default: By default the identify operation identifies the top-most layer (i.e. layers=top)
You can either specify the layer options as mentioned above by themselves or specify the layer option in conjunction with a list of layer IDs. When both the layer option as well as the layer IDs are specified, the server treats it as a boolean "AND" operator. For example, if the parameter is specified as: layers=visible:2,5 only layers with IDs 2 and 5, if visible, are identified.