If you're going to use ConvertToHierarcical.... method in order to convert tabular data into hierarchical, then your source data should be a single table/array/etc.
From my point of view, you should simply bind root nodes to TableA, add a FlyTreeView.NodeDataBound event handler and use it to bind every root node.ChidNodes collection with the required data (from TableB, TableC, whatever).
Note: node.ChildNodes supports DataSource and DataBind() so every node can easily populate its child nodes from datasource.
P.S. Anyway you should use FlyTreeView.ConvertTabularDataToHierarchical(..) method to covert data for root nodes and for child ones.