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I have 3 Update Panels on my page. On Page load, UpdatePanels 2 and 3 are
hidden. When I click a button in UpdatePanel1, I display the other two
UpdatePanels and their contents.

UpdatePanel 2 has an server-side button that when clicked, it needs to
reference controls in UpdatePanel3 and perform some functions on the server.

Since UpdatePanel2 and 3 weren't included in the original page, clicking the
button in UpdatePanel 2 doesn't do anything since the browser doesn't know it
exists.

My question is, how can I have access to UpdatePanel 2 and 3's controls, and
how can I make a server-side call from controls within those two panels?