Solution 1:

The chart is treating the three columns as different series.

Use a formula to copy the data for the selected column into a single column that doesn't change. I have put this formula in G1:

=RIGHT($G$10,1)

And this formula in G2:

=OFFSET(A2:A4,0,G1)

I have list data validation in cell G10, similar to you:

enter image description here

Here, the chart is always using the same range for the data, so it treats it as a single series. As you have added the labels to that series, they are not lost when the data changes.