This means that while the labels of 2 are shown, the labels of 3 are not. How can I 'hide' labels?
UPDATE: To clarify - how do I make the legend automatically disappear when it's a blank series, and appear otherwise?
To add labels one series at a time, don't use the Ribbon Data Labels button. Instead
You can also choose only a single data point to label, if you don't want to label the entire series.
Just single click on the legend text and delete it.
Update Based upon your new question, in your table try
As you can see, Cell A4 will display the series name (in this case Karl) providing all columns have data. The legend will still appear as a colour line, but no text will be associated (not sure if this is good enough).