What chord is formed on the 1st String at the 3rd Fret?

The chord formed on the 1st string at the 3rd fret is G. When you press down on the 3rd fret of the 1st string (high E string), you are producing the note G.

In standard tuning, the 1st string starts as E when open. By pressing down at the 3rd fret, you are raising the pitch by three half steps: E to F (1st fret), F to F# (2nd fret), and F# to G (3rd fret). This makes the note at that position a G note, which is the root of the G major chord.

To further understand this, consider how chords are constructed. A G major chord consists of the notes G (root), B (major third), and D (perfect fifth). If you play the 1st string at the 3rd fret (G), you can combine it with the relevant notes on the other strings to form the full G major chord if you strum those strings as part of the chord. Thus, pressing the 1st string at the 3rd fret directly corresponds to the root note of the G major chord.