In QGIS 2.18, I had been using the plugin "Plain Geometry Editor" to view, copy and paste the WKT geometry of features (as a solution to this question). However, this plugin is not available for QGIS 3.0. Is there any new core functionality within 3.0 that will do the same thing? or perhaps another plugin? ("GetWKT" allows me to view and copy, but not paste, the WKT data.)
In QGIS 3 you can use (like in earlier versions) the copy&paste of the objects in the attribute table. When you paste them you get the WKT and also the other attribute values. If it is not working the option is probably not set in the options (submenu datasources).
That means you have to paste it into another program an copy only the WKT from there if you don´t need more. Spreadsheet software like Calc do automatically recognize the delimiter of the copied data. Therefore you get an own column only for WKT.
I suggest to try OpenJUMP while waiting for Plain Geometry Editor for QGIS 3. With OpenJUMP you can copy, paste, and edit geometries as WKT. QGIS can read and write the JUMP GML format (JML) and you can use that as a transfer format. OpenJUMP can handle all sort of geometries on one layer which is often a nice feature but for QGIS you must keep points, lines, and polygons on separate layers.