Milhouse:
How the Avos davened is not quite so simple. What we do know is that Avrohom Avinu established Shacharis, Yitzchok Avinu established Mincha, and Yaakov Avinu established Maariv. Precisely what they recited is unclear. There is a very interesting source that suggests that their tefilos were quite similar to those we say today. I refer you to the sefer Beis Elokim (written by the Mabi”t – a contemporary and close friend of the Beis Yosef). In Shaar Hayesodos – I think Perek 59 or 60, he inquires about how Yidden davened prior to the codification of the Anshei Knesses Hagedola. He proceeds to state that their tefilos were basically the same as we have today, and describes this in detail. I believe there are other opinions. There are other references that address tefilos of the Avos in the terms of the tefilos we say today. So it’s not quite simple.