Hi Elena Kosinska,
You need to set the HierarchyColumn property of RadTreeListView to the index of the column you would like to show the expanders. In your case, you should set it to 2 to have the third column be the hierarchy column.
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items