I already answered your support ticket, I am pasting my answer here as well, in case someone else has the same problem:
I am afraid that RadComboBox does not support your requirement - the control can use only one property as a source for autocomplete, e.g. it could use only one of FirstName, LastName or ID in your scenario. You could create a fourth property, that will return the combined values of the three properties and switch the TextSearchMode to Contains - this way the control will be able to recognize the other properties, but it will also cease to autocomplete.
Please, let's continue the conversation in only one place.
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