(2) and (3) are both grammatically correct, and both say the same thing. (2) is obviously shorter because it omits the "she's" but since there is no other interpretation for the meaning of the sentence the omission is fine.
(1) is grammatically correct, but here the omission of "about" has created another possible interpretation. (1) could also mean, 'She looks at him as if to say something with the look itself, but then stops, never having intended to actually speak.' Either interpretation is possible, so by omitting "about" you are making the sentence shorter but reducing its clarity.
Thus, if you are trying to reduce length but keep the meaning unambiguously that of (3), your best choice among the three listed is (2).
However, if your goal is brevity, the reflexive pronoun "herself" is unnecessary. Who else would be stopping her from speaking?
She looks at him as if about to say something, but then stops.