Adam Clarke on Jonah 3:4

Yet forty days - Both the Septuagint and Arabic read three days. Probably some early copyist of the Septuagint, from whom our modern editions are derived, mistook the Greek numerals μ forty for γ three; or put the three days' journey in preaching instead of the forty days mentioned in the denunciation. One of Kennicott's MSS., instead of ארבעים arbaim, forty, has שלשים sheloshim, thirty: but the Hebrew text is undoubtedly the true reading; and it is followed by all the ancient versions, the Septuagint and Vulgate excepted. thus God gives them time to think, reflect, take counsel, and return to him. Had they only three days' space, the denunciation would have so completely confounded them, as to excite nothing but terror, and prevent repentance and conversion.