The only differences in these three sections are the maturity of people among ages,understanding humanity,administration, agriculture, spirituality, philosophy and also Unity which divides modern from the past two sections...There was also different situations as everyone has there own perspective of thinking and so that they have different pasts as well..(Now you know,why is it called the book "Our Pasts"?)
You have observed the three stages ( they are actually stages in some means)very closely if you read carefully.....They shows the unknowing of people about unity...All were colonized by different perspectives,castes,religions,genders.
They had forgotten about India..the great land,the real goddess and they fought against each other to make a rule on it..So in this way,The three sections are entirely different from each other.
ancient and medieval were actually the stages of mistakes happened by Indians,but Modern was the like a punishment and atonement to Indians for the mistakes and foolishness they did.

Hope that this will help you....!!