If you created one snk file that you want to use for all three of them, select: Add an existing item from the context menu in projects.
Go to the snk file and select "Add as link" by clicking on the arrow next to the "Add" button.
Then, in the project properties, make sure that on the Signing tab, select the Sign the assembly check box, and then select the key file that you just added to your project.
That should do the trick.
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