Turn on the stream layer.

Use ctrl + mouse left key to select all the three streams. The selected streams will be highlighted in red.

Right click the mouse to open the context menu and select RouteChannel in Generate selected geometry as… menu..

A confirmation message will show up after clicking RouteChannel. Click OK to turn off the message box.

The generated RouteChannel will appear with a different color rather than the imported stream layer.

The length of RouteChannel is calculated automatically. Check parameter values of these RouteChannel in Parameter Tables window located at bottom.