R – Failed to load viewstate

I have an aspx page that dynamically loads user controls: there is an UpdatePanel that holds these controls (one each time). When I change a control with another one it fails with the message:

Failed to load viewstate. The control
tree into which viewstate is being
loaded must match the control tree
that was used to save viewstate during
the previous application. For example,
when adding controls dynamically, the
controls added during a return must
match the type and position of the
controls added during the initial
application.

All of three controls inherit from System.Web.UI.UserControl. Maybe I can solve the problem doing:

1. Load three controls at start time and then hide two of them as we need it.
2. Use three UpdatePanels, one for each user control.
3. Any advice?