“Many congregations pattern their weekly Torah reading cycle after a system similar to the one used in ancient Israel during the rabbinic period. In this system, the traditional parashiot are each divided into three shorter segments, and the whole Torah is completed once every three years. The system has both advantages and disadvantages, but its ability to shorten the length of Torah reading without sacrificing the complete reading of the Torah on a regular basis has made it the choice of some synagogues in the Conservative Movement.”
A Complete Triennial System for Reading the Torah, Committee on Jewish Law and Standards of the Rabbinical Assembly
Update December 2021: In November 2020, the CJLS modified the triennial cycle for some combined parshiyot to change the reading for year 3 to be the third section of the parashah.
Modification of the Triennial Cycle Readings for Combined Parashot in Certain Years, Rabbi Miles B. Cohen