What is a "three-beat song"?

Three beats are commonly known as four-three and eight-three. It is a common beat in music.

The rhythm of three beats is: strong and weak (bang, chug, rub), usually used in dance music, to express the mood of unrest, sounds more beautiful, smooth.

Four-three beats are quarter notes as a beat each measure has three beats the timing of each beat is determined by the tempo of the piece.

Eight-three beat is the eighth note as a beat, three eighth notes per measure.

Expanded Information

In music, time is divided into equal basic units, each of which is called a "beat," or one beat.

The time value of a beat is expressed in terms of the time value of a note, which can be a quarter note (i.e., a quarter note as a beat), a half note (a half note as a beat), or an eighth note (an eighth note as a beat).

The time value of a beat is a relative concept of time, for example, when a piece of music is played at 60 beats per minute, each beat takes up one second, and a half-beat is one-half of a second; when a piece is played at 120 beats per minute, each beat takes up one-half second, and a half-beat is one-quarter of a second, and so on. Once the basic time value of the beat has been determined, notes of various time values are associated with the beat.

For example, when a quarter note is used as a beat, a whole note is equivalent to four beats, a half note is equivalent to two beats, an eighth note is equivalent to half a beat, and a sixteenth note is equivalent to a quarter of a beat; if an eighth note is used as a beat, a whole note is equivalent to eight beats, a half note is four beats, a quarter note is two beats, and a sixteenth note is half a beat.

Reference:

Baidu encyclopedia: music beats