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I notice there's a good bit of space near the draw and abort buttons. Enough room to write, (again permanently so as not to give any hints or require taxing the server to analyse the positions), "To claim a 3 fold repitition, click the draw button."
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Maybe not that...
Ok, but why not that? What would be it's drawback?
For sure not the site staff. There is a page on the site about rules and basics available for everyone interested to study them: http://www.chess.com/learn-how-to-play-chess. There is even a separate page about how to claim a draw: http://support.chess.com/Knowledgebase/Article/View/40/0/how-do-i-claim-a-draw.