Click the Hatch
icon
on the Entities
toolbar or choose Symbols / Hatch
from the main menu and select the Trim/Extend option. The Hatch
dialog appears:

Pick the edges (three) to form the contour of the hatch to be created and exit:

An Open Hatch is created (Default).

To change the chain
to closed, click the Contour button
and click Activate Chain.
The edges forming the contour are highlighted:

Toggle the Open Hatch parameter to Close and exit:

A Closed Hatch is created.