I'll wait and perhaps I won't need a third sensor. My play space isn't that big anyway. And if you look at the two sensor setup of @Mr.Wilde two sensors should be enough if your play space is small enough. Note that the sensors in this build are only at 155cm height. I think you can minimize occlusion if they are hanging from the ceiling. But we will see, time brings the answers.
But how well does that work in conjunction with a desktop experience? Tried putting my camera up in the corner of my room and it kept complaining about not seeing the front of the headset, which means you would have to move it when sitting in front of the screen for desktop games like Elite Dangerous.
I think in the above picture it's calibrated to north and not to the TV (east). Put a sensor on the forward facing wall to prevent the error you mention (maybe with multiple sensors this is not an issue)
if you look at this developers you tube I believe he is showing most of the games that work on the HTC vive with just 2 sensors but he also has videos showing 3 and 4 Reality Check VR