The Shape Tool is F4. You click-drag over the three inner Point Handles and press Delete.
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Thanks for the effort. I'll try that way also. At 1:50 how did you get those lines position in the middle?
If I press Shift+F4 I get points but no handles.
Okay I did doubleclick on the shape then click on the points (I think) then was able to delete points.
Thank you.
ps strange that the corners are not perfectly round but a collection of bended lines.
Attachment 126310
You select the shape first with the Selector Tool (the arrow pointer) then press F4 to enable the Shape Tool.
You are right; it appears to be interference with two co-incident circles at each apex.
Rather than Add Shapes early on, group, clone twice, rotate into position, ungroup and delete the topmost red circles, leaving just one at each corner.
That is far cleaner.
Attachment 126312
Acorn