The most common and most likely answer to this crossword clue is the 5 plus 4 letter word SPACE TIME
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More clues leading to the result SPACE TIME
Buddy explains!

The straight part
We know that "where 3 could be located" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case where 3 could be located = space time.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does SPACE TIME mean?
SPACE TIME noun- The four-dimensional continuum of the three spatial dimensions plus time.
- An n-dimensional continuum consisting of dimensions of both space and time. Normally spacetime is considered as having 4 dimensions (x, y, z, t), but higher-dimensional spacetimes are often encountered in theoretical physics, e.g. the 5-dimensional spacetime of Kaluza-Klein theory or the 11 dimensions of spacetime in M-theory.
- A specific region of the universe with mathematically different properties than the surrounding spacetime. Synonymous with "metric" within the context of general relativity.