Describe the feature

On the left of the image is the three-way corner but it has a raised point in the middle, so I tried to fix it with collapsible framed blocks but it left the sheet at the bottom, can you please add a flat three-way corner framed block to 1.20.1

If you mean the shape shown in the following screenshot, then that already exists as the Framed Prism Corner:

Due to lack of response I'm going to assume that the block suggested above matches what you were looking for.