Creating the union of the main pipes

Choose the Zoom->In option from the mouse menu. Magnify the right end of the model and rotate the view to facilitate the selection.
Set "union" as current layer to use, with a <Double-click>
From the Copy window, select Lines and Rotation. Enter an angle of 120 degrees and select the rotation center (160, 25) as First point. Since the rotation may be done in 2D, choose the option Two dimensions. From the Do extrude menu, select Surfaces and be sure that Multiple copies is 1 and Maintain layers is unset, then the new entities will be created in the layer to use instead of the layer of the source curves.

The lines to be selected

Click Select and select the four lines that define the right end of the pipe (see figure above). Press ESC when the selection is finished.

Result of the extrusion by rotation

GiD user manual