It is more of history than religion! Whole of this Shaivya, Brahmana and Vaishnav concept. They are not gods but sects and sect leader for every time of need. No one remains same as all that is born has to die, universal rule, but the sect so honestly and efficiently transfers the knowledge to every member of brotherhood that at any stage there is someone to take place of previous Shiva, Brahma and Vishnu. To define in modern terms they are the best of the laureates and scientist, mastered in every kind of art and knowledge. Everyone has a role, Vishnu taking the world in stomach is more of a reference to fact that he saves whatever could be of physical world so that when situation gets better a restart can be arrange, from varha avatar to Noah's ark you find a reference of world being saved from drowning. It is more of reference of surviving through bad times. Shiva's unaffected figure is actually his excellence at fighting off danger and killing evil, he is a destroyer but not universal, more of everything evil, even perception and myth. He is a figure of true physicist who talks on rule, makes equations, burst myth and yet can make a nuclear bomb and can turn everything into dust. Vishnu is the holder of knowledge of everything required to live, cultivation, housing, construction, reproduction, growth etc and Brahma is the philosopher, thinker, law giver, he mostly sets the rule for things to get rolling.
I hope this helps. A simple request, we belong to a great past and I don't only mean a particular country or religion, the whole wide world and the religious scriptures are the only sneak peek we have left with us for the really old forget past. Do not read them in complete devotion and blind faith, read them with open and inquisitive mind and then instead of myth you will be reading history. In their current form they have gone through multiple language change and interpretations. Try not to read them literally but try to get their spirit and find science in them.