I gave it a try on a dummy report in Northwind. It did work, so maybe I wasn't clear enough on what I did.
To make it work, the Combo needs to be in the Detail section of the report -- therefore, a different value for each record. If you put it in a header section you may strike problems.
Here's a quick summary of my test:
In Northwind, create a new report using the wizard. I chose Orders as the record source for the report and selected CustomerID, City and Country as the fields. No sorting or grouping, Tabular layout in Portrait mode.
Note that CustomerID is already a ComboBox. I went into the CustomerID combo and edited the RowSource, adding ContactName as the third field. Closed and Saved.
Now add a new Textbox to the Detail section of the report. Set its ControlSource to be
=[CustomerID].Column(2)
Select Enter to save the changes. Run the report and you should see the contact name appearing in the report. Yep, I cheated for simplicity and used the same recordset but I have built reports and froms before where the combo brings together data from up to 3 different recordsets.
Good luck