The 3rd pattern A Major scale uses the G shape open chord. When looking at the open G chord, the root notes are on the open G string and the 3rd fret Top and Bottom E strings.
G
Important: The notes used in the A Major scale are A B C# D E F# G#, there are three sharps in the A Major scale.
If we take a look at the scale on the fretboard, you will see something like the following pattern… the root notes are marked with the “X and the thick black double lines represent the nut.
| || $3.1 | $1.2.$2.2.$3.X.$4.2.$5.2.$6.2 | $2.3 | $1.4.$3.4.$4.4.$5.4.$6.4 | $1.X.$2.5.$5.5.$6.X |
Note: In the diagram above it’s the fret numbers that are indicated not the scale degrees, and it isn’t the way you would normally write out a scale using TAB, it’s just to illustrate the fingering pattern of the scale. 
When playing this scale on the guitar, I normally start on the lowest pitch root note, the 5th fret A on the Bottom E string, and play up to the highest pitch note of the scale, 1st string 5th fret, which is also one of our A root notes.
| $6.5 $5.2 4 5 $4.2 4 $3.1 2 4 $2.2 3 5 $1.2 4 5 |
Descending back to the lowest pitch note of the scale, the F# on the 2nd fret Bottom E string.
| $1.5 4 2 $2.5 3 2 $3.4 2 1 $4.4 2 $5.5 4 2 $6.5 4 2 |
Then back up in pitch to the A root note on the 5th fret Bottom E string.
| $6.2 4 5 |