All three.
None of the oldest sources seem to confirm or deny, since Medousa's sisters don't really do much in the most famous myths. But some later authors made it explicit.
From Nonnus's Dionysiaca, from the fourth or fifth century CE.
ἢ Περσέος εἶχες ἀγῶνα;
ἢ Σθεννοῦς ἴδες ὄμμα λιθώπιδος ἠὲ καὶ αὐτῆς
δύσμαχον Εὐρυάλης μυκώμενον ἀνθερεῶνα;
Have you had Perseus's quests, then? Have you seen Sthenno's petrifying eye? Have you heard the screaming mouth of Euryalë herself?
(Book 30, lines 264-266, translation mine)
A bit earlier, Pseudo-Apollodorus (author of the Bibliotheca) says in the first or second century CE that all three could petrify:
ἦσαν δὲ αὗται Σθενὼ Εὐρυάλη Μέδουσα. […] τοὺς δὲ ἰδόντας λίθους ἐποίουν.
[The Gorgons] were Stheno, Euryalë, [and] Medousa. […] They turned anyone who saw them to stone.
(Translation mine; I can't seem to find an edition with line numbers to cite.)
I can't find any other sources that mention Sthen(n)o and Euryalë being able to petrify, or not. As Lauren mentioned, Homer and many other authors mentions only a single Gorgon (sometimes "Gorgo"), whose head was on Athena's shield; the sisters don't show up much outside the Perseus myth.
Apart from the petrification, Hesiod similarly mentions that all three were monsters; Pseudo-Apollonius says all three were identical in appearance; Ovid says that all three were originally beautiful, but Medousa had her hair turned into snakes. Medousa's defining characteristic seems to have been that she was mortal, while her sisters weren't, which is why Perseus was able to kill her:
Σθεννώ τ’ Εὐρυάλη τε Μέδουσά τε λυγρὰ παθοῦσα·
ἡ μὲν ἔην θνητή, αἱ δ’ ἀθάνατοι καὶ ἀγήρῳ,
αἱ δύο· τῇ δὲ μιῇ παρελέξατο Κυανοχαίτης
ἐν μαλακῷ λειμῶνι καὶ ἄνθεσιν εἰαρινοῖσι.
[There lived] Sthenno, and Euryalë, and Medousa, who suffered such misery. Only she [Medousa] was mortal, while the others are undying and unaging, the two of them; she [Medousa] is the only one blue-haired [Poseidon] wandered with amid the soft meadows and springtime flowers.
(Hesiod Theogony 276-9, translation mine.)
This is the only mention I can find of multiple Gorgons in the Theogony, with no mention of petrification. Many authors specify that Medousa could petrify people, but I can't find any specific mention that her sisters couldn't.
