I was wondering whether you can choose a pair of gpus to render in a three gpu machine. Maybe a checkbox-like option in which you designate which graphics cards to use. All that Blender is allowing is either using all three cards or just one.
Yes, when you have more than 1 GPU blender options are all the GPUs at once and then each single individual GPU.
This feature is on the dev's todo list for longer than 2 years.
You can check it here: https://developer.blender.org/T37717
Dec 8 2013, Brecht wrote:
It's on my todo list to improve graphics card configuration for 2.70, to allow you to select individual cards and enable/disable them. This is not considered a bug at the moment however.