ing clause

Is (1) considered as a shortened form of (2)/(3)?

(1) Whose is a possessive relative word, referring to people and things.

(2) Whose is a possessive relative word, which is referring to people and things.

(3) Whose is a possessive relative word, refers to people and things.

(3) is incorrect.

(1) and (2) require inverted commas around ‘whose’ and (1) is the most natural form. (2) sounds awkward, and would be better if you used a different tense:
‘Whose’ is a possessive relative word, which refers to people and things.
(1) is a correct form in its own right, it is not a shortened form of (2).

The term ‘possessive relative word’ also sounds odd and I suggest ‘possessive relative pronoun’ instead.

Thanks.Is (1) considered as the reduced relative clause of (2)?

For example:
Do you know the woman who is talking to Tom?
Do you know the woman talking to Tom?