Brahma is God who creates all forms, hence is called the creator; but he has not yet found the perfect form and is still yearning and searching, making him unworthy of worship. Vishnu is God who has realised that no form is perfect and so works with the limited forms. This is why he is called the preserver and is worshipped in various forms. Shiva is God who breaks free from all forms, having found all of them limited, hence he is the destroyer who is worshipped as the linga.
