Safir is a fine, whistle-like sound produced with certain consonants. It is a characteristic resonance, not a requirement to add an exaggerated hiss or a separate artificial whistle.
Its three letters are ص، ز، س. All have safir, but their remaining features differ: ص has istiʿla and itbaq, while ز and س do not.
Contrast: recite «أَسْ», «أَزْ», and «أَصْ» distinctly. The presence of safir in all three does not make their full pronunciation identical; the complete set of qualities determines each sound.