How to play a major C chord in its open position?

There is a simple technique that music theorists use in order to achieve the major chord quality.  This is a formula given in 3 notes.  The 1st note, the 2nd note, and the 3rd note of the chord. 

The 1st is any note you want...
The 2nd note is 4 half steps away from the 1st note (M3 or "major third" interval away)...
The 3rd note is 3 half steps away from the 2nd note (m3 or "minor third" interval away)...

Guitar example:  Major C chord is constructed with:
The 1st is any note you want...= C (5 string 3rd fret)
The 2nd note is 4 half steps away from the 1st note (M3 or "major third" interval away)...= E (4 string 2nd fret)
The 3rd note is 3 half steps away from the 2nd note (m3 or "minor third" interval away)...= G (3rd string open)

So why does one play the other strings when fingering a C major chord (the 2nd and 1st string and even the 6th string???)...

Well, the 6th open is ok to play since is E and forms part of the chord anyway...and the 2nd string is played in the 1st fret which is C...and the first string is played simply open as part of the chord as well since it is E.

So there you have it, the formula and the way to finger an open C chord!