If you played the third fret on your top most string,that is called "G" in notes.

A "G" chord would be "GDG",played on three strings at the same time

3---
5---
5---

But if you take your top most string,all the notes for that would be,going up on fret at a time:
E(open string),F,F#,G,G#,A,A#,B,C,C#D,D#,E(12th fret)

Sight reading musical notation takes a bit more practice.