Round somes corners only I would like to round corners (surrounded in red) of this shape:



I tried some options:


ctrl+shift+B : this does not work on the surrounded vertices.
Subsurf. It look like I can use this technique but it increase the polygons significantly. But if this is the only solution I'll use it
Temporarily extrude edges

You’ll be able to round the corners with the bevel tool (Ctrl+B) by extruding the boundary loop just to feed Blender with some faces to work with and then deleting the extrusion.