The notes you list are for the C major scale starting at A.

The A major scale is:
A, B, C#, D, E, F#, and G#. Look up how the major scale is derived and this will make sense. Wikipedia is a good source (it's where I found quickly the notes of the A major scale). It is defined by intervals starting from a root note.

The A major breaks down like this:
1 - A
2 - B
3 - C#
4 - D
5 - E
6 - F#
7 - G#

So, the 1st, 3rd, and 5th of the A major scale are A, C#, and E. Flat the 3rd and you get C.

This all has nothing to do with which string we are talking about or in what order they are strummed. You can use this idea to play an A and an A minor chord on a piano.

Hope that helps! It actually helps me trying to explain it to somebody else :-).

Rich