Who are the ancestors of the Israelites?

I don't think the earnest beginnings of the Israelite people should begin with Abraham. Although from Abraham a great number of people came and a great nation was created, I would say the most definitive top of the tree is Terah. It is from Terah's children the real division of MOST ALL the tribes of middle east area came to be. Specifically, Abraham's (traditional 12 children) divided up the "promised land", but they often married and were punished for marrying "outside" the faith of Abraham's God. Those non-Abrahamic/Israelic tribes are many of the tribes that must be included in the settlement of the area.

The best example of this is Terah's son, Haran. Haran had a son Lot and it was through Lot came the Moabites and the Ammonites. To make my point additional clear, there were, of course, the 12 tribes of Israel that divided up the promise land but keep in mind... there were SIX ADDITIONAL sons of Abraham through his last wife Keturah. The most popular of the six additional sons was Midian who it appears to have settled east of the Gulf of Aqaba giving the ancestral background to Midianites and to Jethro and the future Mrs. Moses.

Trying to find evidence of the existence of 3 people outside of their own version of history or cultural narrative essentially means that those 3 people would have to be famous outside of their own lineage. They would have to interact with a prominent member of another culture in such a way that was historically significant for that other culture to document.

So if your own people mostly know you as result of having children that wouldn't qualify as a reason for another culture to document that person's existence.

Archaeology of the Hebrews

Most scholars tend to agree that either these stories did not happen or they were overblown in order to establish an inspirational history that made them look good. Richard Elliott Friedman also seems to agree that the lack of evidence may simply indicate that stories are embellished, but that doesn't mean there is no truth to them. If you're talking about 2.4 million people in the desert that's different from the type of evidence you'd find with a much smaller group. Also there is a lot of evidence that different parts of the story were taken from Egypt and Canaan. Many details should not be held against the writers because much of this early history is taken from oral traditions.

The Exodus is not Fiction

It is of course impossible for the Israelites to have no ancestors. It is also impossible to know their ancestors with absolute certainty. I give you here several quotations from "A History of the Jewish People", chapter 3 "The Dawn of Israel" by Abraham Malamat, edited by H.H. Ben-Sasson, from Harvard University Press to provide a modern historical answer.

"The genesis of every nation and tongue is enshrouded in obscurity, and generally there survive only a few vague recollections of limited historical value. Israel alone among the nations of the ancient Near East has preserved any organic, ramified tradition - as exemplified by the Pentateuch and the Book of Joshua - recounting its origins and vicissitudes prior to its crystallization as a true historical entity."

"A cardinal question immediately poses itself and is the basis for any proper assessment of the historical beginnings of Israel: how is the biblical tradition (or, more precisely, the biblical traditions) to be evaluated from the standpoint of historical authenticity? The problem applies to the historical sketch in its broad outline as it emerges from the biblical account: the origin of the patriarchal family in Mesopotamia and its migration to Canaan; the social and religious modes of life followed by Abraham, Issac, and Jacob; the bondage in Egypt and the subsequent Exodus; the desert wanderings and the ultimate conquest of the Promised Land. Can this entire account or even a portion of it be viewed as faithfully mirroring historical reality?"

The book then goes into contrasts of various schools of thought from the radical denial of the biblical tradition to a blind respect for it, concluding with "In the subsequent sections, we shall employ a dialectal approach to the biblical material - in contrast to the one-sided radical methods noted above."

After discussing the difficulty of dating the Exodus we read: "Attempts to determine a comparatively accurate date for the Patriarchs are themselves doomed to failure, for in fact it is difficult to speak of the so-called 'patriarchal period' as a well-defined chronological entity, even where one accepts the biblical tradition as such. It would seem, rather, that imbedded in this narrative cycle are reminiscences of centuries-long historical processes that may hark back to the West Semitic migrations within the Fertile Crescent that made their way ever westwards and reached their apex during the first quarter of the second millennium [BCE]. These extended time spans were telescoped in the biblical narrative into a mere trigenerational scheme - Abraham, Issac, and Jacob."

Later we can find: "The Aramean element in the patriarchal stories is seemingly a later anachronism. There is thus no basis for the current scholarly contention that the Israelites were of Aramean or 'proto-Aramean' extraction. The Hebrews are, rather, to be linked with an earlier West Semitic stratum known in scholarly terminology as the Amorites (derived from the Akkadian designation 'Amurru', to be distinguished from the biblical usage of the Amorites), who first appeared in the Fertile Crescent towards the end of the third millennium [BCE]."

There is much more, bringing in extra-biblical material such as place names and archaeological finds, but the above is the crux of your answer. For more details, I recommend you get this or a similar historical work from the library.