Cascading Dropdown; Getting value of ContextKey

I need help on how to get value in the context key.

Below is my html code

<ajaxToolkit:CascadingDropDown ID="ccd4" runat="server"
      ServicePath="~/Vehicles.asmx" ServiceMethod="GetEngines"
      TargetControlID="DropDown4" LoadingText="Loading..."  ParentControlID="DropDown3" Category="Engine" 
      PromptText="Select Engine" UseContextKey="true"  ContextKey='<%# Bind("DropDownn1") + "," + Bind("DropDown2")+ "," + Bind("DropDown3")  %>' />

 In my web method I need the split the context key string to get single value of dropdown 1, dropdown 2 and 3 but I dont know how  to get the context key string.