Closed corner and Three bend corner are used in different aspects. Here one is used for the two flange corner  and other is used for three flange corner.

Closed corner:

closes a corner where adjacent two flanges meet by extending  the bends and flanges. Here to perform the closed corner operation .Initially draw the sketch of rectangle are something.

Then convert the sketch into sheet body by using tab tool. Now make flange on each side of the body then apply the tool over required area.

Fig shows the application of tool over body.

In this we have different options to apply as per the requirement.

Treatment,overlap,gap.

Three bend corner:

closes a corner where adjacent three  flanges meet by extending  the bends and flanges.

Initially draw the sketch of rectangle are something. Then convert the sketch into sheet body by using tab tool.

Now make flange on each side of the body then apply the tool over required area.

Fig shows the application of three corner tool over body.

In this we have different options to apply as per the requirement.

Treatment:open,closed and circular cut out.

Here is the video for better clarity.