HINDUISM is monotheistic. It teaches that God who is omnipotent, omniscient, and omnipresent is one and only one.  “God is one and only one; the wise called Him by different names” (Vedas). We believe that it is from God all things originate and were begotten; and from whom, having been born, they live; and into whom they finally enter. It therefore means that Hindus must understand that the three names; Brahma, Vishnu and Shiva are not three different Gods, but one having three different names, and is called by these different names when He performs different works. When He created the universe, he is called Brahma; when He entered in the universe to protect it, he is known as Vishnu; and when He caused its dissolution, He is looked upon as Shiva.
Hindus must never mistake the philosophy of Hinduism for mythology. We must understand that mythology has its own significance and purpose. Mythology is there to guide man on the path of religious and good living. It depicts certain principles of life through stories; legends; dialogue; lives of great Kings and renowned Sages. It is intended to help those who cannot comprehend the philosophical teachings of the Vedas to get an understanding of the same teachings through stories, dialogue, etc. It helps them to live good and pious lives and to follow the foot steps of the characters of the mythological works. In mythology, the characters are faced with predicaments and the poets carve out ways base on the teachings of the Vedas to solve the dilemmas. So, when the ordinary people encountered similar problems in their lives, they will be able to find pious and non-violent ways to solve them.
However, it must not be denied that our real mythology works have been tampered and perhaps were rewritten during the many invasions by foreigners into Bharat (India). Many forces tried to wipe out the existence of Hinduism from its own land and have destroyed and rewritten our books to make Hinduism appear “backward”. That is why many works of mythology bearing the name Hinduism do not really convey the teachings of Hinduism, and even some of these stories and legends are unbelievable and create doubts and confusions amongst its readers and listeners.
Hindus, therefore, must not be confused by any and all things they hear and read. They must understand that every religion is made up of philosophy, mythology and rituals, and simultaneously they should try to understand the differences, symbolism and intentions of each.
The idea of trinity i.e. Brahma, Vishnu and Shiva are therefore one God having different names and attributes. “He who is known as Vishnu, is the same Shiva; and he who is Shiva; is Brahma” (Upanishad). It is like one person performing different duties bearing different names for each duty, for example, a person can be a child to his parents; a brother to his siblings; a husband to his wife; a student to his teachers; a teacher to his students; maybe a gardener, a painter, etc, in his spare time. For each aspect he is sometime called by different names, but he remains the same being. Similarly, God is called by many names, but He remains the same One Unchangeable Supreme being.