CHORD CONSTRUCTION works like this for basic triads.
The 1 is the root of the chord. (lets say were talking about a c maj chord here, the scale the chord is derived from would also be c major)
the 3 stands for the 3rd of the chord, or 3 notes above the root (wich in this case would be E)
the 5 stands for the 5th of the chord, or 5 notes above the root.(G)