During a request for Items you can only load new Items in the RadComboBox. Changing properties of the control or other controls on the page will not have any effect on the client.
In your case, you could handle the client-side ItemsRequested event of the third RCB and there preselect the 'Paris' Item if it exists. The approach is similar to that in the demo you referred to.
I hope this helps.
the Telerik team
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