The third aliyah continues the promise of G-d begun in the previous aliyah that if we observe the mitzvahs we will dwell in the land of Israel in safety. This aliyah adds that the land will produce its fruit to our satisfaction, and adds again that we will dwell in safety. It then promises that we need not worry about what we will eat during the Shmittah year when we can't farm, for the sixth year will produce enough for three years (the sixth, seventh, and eighth years).