Vrish,
What you are saying is true. But when we read the different stories from the puranas, it is important to see from which POV it is written. For instance, in Vishnu Puran, Srimad Bhagavatham, 4000 Divya Naama Prabandangal (a kind of religious - poetic work in Tamil in praise of Vishnu) Vishnu is considered Supreme.
In Shiva Purana and other works in praise of Shiva, Shiva is considered superior to Vishnu and others. In such cases, he is considered Creator, Preserver and Destroyer. In fact the five faces of Shiva stand for the five functions he performs- Creation, Preservation, Dissolution, Concealment and Grace. According to the Panchanana Stotram, the four Vedas originated from four faces of Shiva while the fifth face is the source of OM.
The definition of Brahma as Creator, Vishnu as preserver and Shiva as destroyer seems to me to be a very shallow one. In several cases, as you have mentioned above, each god has performed multiple functions.
Moreover, the traditional Trinity definition eschews the role of Devi .In the scriptures related to Devi, she is considered the Supreme Power. In fact in one slokas in her praise, it says that the ten avatars of Vishnu were created through her fingernails (karanguli nakhotpanna narayana dashakrithi).