How do I specify the mouse button (mousebuttonNumber) in a script?

[Dr. Robert E. Ball]

I want a mouseUp handler based upon button 3. In the printed Transcript
Language Reference and the dictionary the mouseButtonNumber is indicated by
"€3" (Option + *). I have tried that in the form

On mouseUp €3

but that doesn't work. How can I indicate button 3 on the mouseUp handler?