Basing himself on the passage in Sefer Tehillim (1:3) Rabbi Tanhum bar Hanilai taught that Torah study should be divided into three parts, and a person should divide his years so that one third of the time is spent studying mikrah (the written Torah), one third studying Mishnah (oral traditions) and one third Gemara (underlying concepts and discussion of the Mishnah). The Gemara objects to this suggestion, arguing that a person cannot possibly know how long he will live, and will not be able to divide up his time properly. In response the Gemara concludes that a person should divide up his days, rather than his years.
Rashi understands the suggestion of dividing days to mean that the days of the week should be devoted to different areas of study. Tosafot disagree, and rule that every day should be divided up. This appears to be the source for the Ge’onictradition – one that appears in our prayer books to this day – that includes korbanot, a section of readings culled from the written Torah, the Mishnah and the Gemara, whose focus is on the daily sacrifices. Traditionally, people rely on a different suggestion raised by Tosafot in the name of Rabbenu Tam, that the standard Babylonian Talmud includes a mixture of mikrah, Mishnah and Gemara, and its study fulfills the requirement of dividing the days between these different areas of Torah study.
In his Lekutei Torah, Rav Shne’ur Zalman mi-Liadi suggests that the categories should be viewed more broadly, and that mikra refers not only to the written Torah, but also to the midrashim and commentaries written about it, while Mishnah refers to the halakhic part of the Torah. This allows a person to keep a schedule of dividing Torah into three parts even as his develops intellectually and needs less time for “simpler” aspects of Torah study.