Two Nvidia GTX cards in SLI + another heterogenous one

There are tons of answers in regards to using two identical, or unidentical cards in SLI. However I'm having trouble finding out if I can use two identical cards (gtx 970) in SLI in addition to having yet another GTX card not identical to the SLI setup.

I want to upgrade and buy a 1080 when they're out, and I'm wondering if I can dedicate the SLI to physX while using the 1080 for rendering. I realise this might be overkill though.

So can I even do this with Nvidia cards? If yes, should I use the cards this way, or should I sell one 970 and only use one for physX?

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