Sorry, I work in IT and I get a lot of really simple questions; you'd be surprised by how stupid can be. I've had people ask why something does't work. It's because it wasn't plugged in. Some people couldn't log in. Because they didn't spell their own name correctly. So just kind of force of habit for me to go to the stupidest answer first. Sorry.
The third hole is probably the proximity sensor. That's been there since the original iPhone. Basically, if you put your phone next to your ear, the proximity sensor tells the phone to turn off the display so you don't accidentally press something.
I know I work in IT and had to spend about 1 hour explaining how to use a scroll bar!
Yes but it seems odd having a proximity sensor off centred? plus i'm sure the proximity sensor of the previous iPhones are long like the mic and not a small circle? plus not visible in black models?