In a stave
Two notes with one free space or line in between them form a third. Depending on where the third is located in a stave it can be a minor third (m3) or a major third (M3).
On a keyboard
When two notes are three semitones (or keys on a keyboard) apart, they form a minor third. When they are four semitones apart, they form a major third.
Examples
Brahm's Lullaby
Swing low, sweet chariot
