Seforno: Of course she believed. The problem was, that she assumed the angel's words were merely a prophetic blessing, ¹ which, under the circumstances, she thought, could not possibly materialize. To have children at her age would be akin to Techi'as ha'Mesim, which could only come about directly from HaSh-m.

Ramban (to 18:15): Although the angels came in the guise of men, Avraham, in his wisdom, recognized them as angels of G-d, Sarah (like the wife of Mano'ach in Shoftim 13:6) did not. Perhaps she did not even see them.

Moshav Zekenim: She initially heard on Pesach, and laughed six months later, in Tishrei, for no change had occurred. She thought that there was no longer sufficient time to give birth in Nisan, for she did not know that it would be a leap year with an added month. Also refer to 17:21:151.1:2.

Pirkei Hadrachah l'Ven Torah p.174-176: She assumed that he is an Aravi, suspected to worship the dust on his feet. Even so, she should have considered as if his words came from Hashem, for His Hashgachah directs all that we see and hear!