"Then God said, 'Let the land produce vegetation: seed-bearing plants and trees on the land that bear fruit with seed in it, according to their various kinds.' And it was so. The land produced vegetation: plants bearing seed according to their kinds and trees bearing fruit with seed in it according to their kinds. And God saw that it was good. And there was evening, and there was morning - the third day." NIV translation
Throughout God's word, the number three is associated with the earthly display of God's will and we see that here. God not only created plants and trees but also had them to produce seeds so that they could continue to provide for the needs of the animals and people that will be created. In this way, we see that it has always been God's will to sustain His creation.
"And God said, 'Let there be lights in the expanse of the sky to separate the day from the night, and let them serve as signs to mark seasons and days and years, and let them be lights in the expanse of the sky to give light on the earth.' And it was so." NIV translation
Without light, the life that was created on the third day could not be sustained. This reminds us of the fact that sin and the fall of man did not take God by surprise. He knew that He would not dwell with man for long on this earth and, since God is the source of light and He loves us so much, He created lights in the sky to sustain us in His absence. This is only temporary as we see, in Revelation 21, that once again God will dwell with men and the lights in the sky will no longer be necessary.
"God made two great lights - the greater light to govern the day and the lesser light to govern the night. He also made the stars. God set them in the expanse of the sky to give light on the earth, to govern the day and the night, and to separate light from darkness. And God saw that it was good. And there was evening, and there was morning - the fourth day." NIV translation
Throughout the Bible, the number four is associated with trials. On this day, God created the sun and the moon and gave them a threefold purpose (to give light, govern, and separate). We looked at light in the last passage. They govern in that, for much of time, what we did as people was determined by the time of day. (People used to work only during hours of sunlight because there were no other lights such as electric or gas powered lights) To this day, many farmers still plant and harvest by the phases of the moon. Their third purpose was to separate light from darkness. Light and darkness have nothing to do with each other and in fact cannot exist together at the same place and time. God is light and, as Christians, we walk with Him in that light but many times we take a wrong turn and end up in the dark. That usually happens when we are faced with trials and temptations (symbolized by the number 4).