But: if this is a from a larger set, and the answer length is given as 3 letters, then "by 3" means the word is 3 letters long, and the preceding text gives the wordplay.

The string appears to be a cryptic clue in the style of a cryptic crossword.

Given the ambiguous nature, I think this is a cryptic instruction: is an anagram of "two by dy" = two by dy? dy = dysprosium. No.

However, if the dash before "piece" means the answer is hyphenated, "three-piece" fits, but the clue gives 3 letters. Could it be ?

But without more context, the cleanest match to a 3-letter "piece" is (as in a piece of something) — but the wordplay "twbydy" doesn't yield that.

(since it says "piece" in the prompt): "TRI" or "TRIO" ? But length 3 → "TRI" .

Given the obscure nature, I’ll answer: