The IPA symbol is not the digit ⟨3⟩ or the Cyrillic small letter Ze (з).

The value was specified only in 1993; until then, ⟨ɜ⟩ was an alternative symbol for the mid central unrounded vowel [ə].

The ⟨ɜ⟩ letter may be used with a raising diacritic ⟨ɜ̝⟩, to denote the mid central unrounded vowel.

It may also be used with a lowering diacritic ⟨ɜ̞⟩, to denote the near-open central unrounded vowel.

Conversely, ⟨ə⟩, the symbol for the mid central vowel may be used with a lowering diacritic ⟨ə̞⟩ to denote the open-mid central unrounded vowel, although that is more specifically written with an additional unrounding diacritic ⟨ə̞͑⟩ to explicitly denote the lack of rounding (the canonical value of IPA ⟨ə⟩ is undefined for rounding).