These are no ellipses (commonly written with three consecutive dots ...) but three consecutive accent grave
``` (as opposed to ...)
So done correctly your code would look like this
Serial.println("With ```cpp");
and
Serial.println("With only ```");
or
Serial.println("With ```text");
But you are actually using the correct symbols (accent grave) - the problem tho' is that you also have extra text following the tag (i.e. a blank).
The special format tags need to be all alone on their line without any extra characters (visible or not) leading or trailing.