A major chord triad is always built by: Root Note + 4 semi tones + 3 semi tones
A minor chord triad is always built by: Root Note + 3 semi tones + 4 semi tones.
Another way to view this is:
To move from major to minor, lower the 3rd by a semi-tone
To move from minor to major, move the 3rd up by a semi-tone
Let me know if this helps!