They are often considered to be the three most important gods. In modern day Hinduism, Shiva and Vishnu have far more followership, I guess you could say, or more people view Shiva or Vishnu as the supreme being, and there's more temples dedicated to Shiva and Vishnu than there are to Brahma. Followers of Shiva, those who view him as the supreme being, they're called Shaivas, they're following Shaivism, or sometimes referred to as Shivaism. Shiva has multiple aspects, and as we'll see, many of these gods have multiple aspects and multiple connections, but is often referred to as the destroyer or the transformer. Vishnu, as I mentioned, also has a significant following. A significant chunk of Hindus are Vaishnavas, or followers of Vaishnavism or Vishnuism, and Vishnu is considered the preserver. Brahma, as I mentioned, does not have as much followership in modern day Hinduism, but he is considered to be the creator.