When the TriStateCheckBoxes="true" property is present in the TreeView configuration, then checking the child node would automatically set the parent node's state to Indeterminate. This state is then available through the CheckState read-only property of the Node.
With that said, if you need to set Indeterminate state of a node, you just need to have at least one of its child nodes to be checked and the TreeView will mark it Indeterminate itself.
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