I think I figured it out. Go to the dropdown menu for VOICES, and select #3. Then click on that text option on the far right that has the grey L in the background, that manages lyrics text. Then carefully hover your curser over the first note and move it down to just below your second line of lyrics (or Voice #2, as the program seems to view it). a dotted line should follow your curser from that note, (as it had done with your first two lines), so you know you've established a connection to a third line! proceed to type in your words as you already know how.
What I am trying to figure out is how to number the lines, so people don't get muddled when they go down to the next stanza....