<b><i>I have been in this forum for a while.</i></b><br/><br/><b><i>I have been on this forum for a while.</i></b><br/><br/><b><i>I have been at this forum for a while.</i></b><br/><br/>I am working on the above sentences for a while, but still I cannot come to conclusion as to their exact meanings.<br/><br/>I know, all the above three sentences are correct grammatically and semantically though they differ in meanings each other.<br/><br/>Could you help me what the each sentence exactly means above?