The three discoveries are that
1) The cosmic background radiation is a perfect black body spectrum with a unique temperature of 2.7 Kelvins. There are no distortions in this spectrum which means that there were no additional sources of radiation in the universe except the cosmic fireball itself dating from a time of 1 year after the Big Bang.
2) The radiation is uniform in brightness to better than 1 part in 10,000 but there is a 'dipolar' pattern present below this due to the motion of the Earth, Sun and Milky Way relative to the background radiation.
3) At a level of a few parts per 100,000 there are large-scale irregularities present in the background radiation which have amplitude and intensity distributions that also match theoretical predictions for the origin of clumpiness ( galaxies and super clusters of galaxies) in the present universe. Several of these features have been confirmed by independent experiments.
COBE has confirmed key predictions made by Big Bang cosmology, and has shown which new directions research now needs to take to connect the COBE epoch at 300,000 years after the Big Bang, with the onset of galaxy formation being studies by the Hubble Space Telescope at about 1 billion years after the Big Bang.