I can't find the "stamp" block in Scratch 3. Can't also find most of the blocks with the pen symbol
Pen blocks ('stamp' is one of those) are a separate extension in Scratch 3.0. Please follow these steps to access these blocks.
You should now see 'Pen' as a separate category in the toolbar along the left-hand side of your window. Within that category, 'Stamp' is the second block from the top.
You only need to do this once per project.
During the migration from Scratch 2.0 to Scratch 3.0 (January 2, 2019), the pen extension was automatically added to every existing project on the Scratch website that contained pen blocks. So no need to add the extension yourself there.
See also: