I think you should add a team chat window in doubles and triples game so you can discuss strategy openly without the other team seeing the messages. This would work a lot better than constant private messages...
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None of you understand. I'm not locked up in here with you. You're locked up in here with me.
And until then, if you don't like PMs I'd suggest using some sort of external chat feature. There are many out there. And if you don't wish to download anything, if you have G-mail or can get someone to invite you, it is an internal chat in the browser.