What is made up of 4 letters, still made up of 5, occasionally written with 12 letters and rarely with 6, but is written with 3 letters at none of the occasion

Trick Mind Teaser

The statement is true.

What as 4 letters.
Still has 5 letters.
Occasionally has 12 letters.
Rarely has 6 letters.
But has 3 letters.

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