There are two notes to learn on the third string, the open G note and the A note on the second fret.
The open third string is a G note. This G note is written on the second line of the staff.
Exercise 15 introduces the open G note.
The note on the second fret of the third string is an A note. Use the second finger of your left hand to fret this note. This A note is written in the second space of the staff.
Exercise 16 introduces the A note on the third string.
You now know eight notes on the first three strings.