The 3rd one is used for noise cancellation so I suppose I will have to ask people that I speak to on the phone whether they can hear me clearly. My sister's iPhone has incredible noise cancellation; sometimes she calls me from a busy street and I can hear her clearly but barely hear the nearby traffic.
I tested the phone today on a call to my sister. She had no difficulty hearing what I was saying, though I was indoors at the time so I wasn't testing the noise reduction system.
I will phone a pal later, with music playing across the room in the background. I'll ask her to tell me how loud the music sounds relative to my voice.
I have the phone in its new case. I tested the phone on speaker, held directly against my head, and held against my head with the case closed. In all 3 tests, my voice could be heard clearly and I could clearly hear what my sister was saying.
The one slight negative on this phone is the quality of the speakers compared to the great speakers in my old phone. The speaker on the bottom of the new phone is conventional; its sound coming out of the slot that I had described in an earlier post. That is about 75% of the quality that I am used to. The earpiece speaker at the top of the phone is only about 40% of the quality though. It is an interesting design, which does not sound through a hole in the case; it actually vibrates through the screen! I'm not sure why they couldn't have replicated the bottom speaker on the top?

There is no bass from it so it sounds a bit tinny. Perfectly usable audio from the phone though. If I want quality I will listen on headphones or my TV soundbar via bluetooth.
So... I have an excellent phone for about 30% of the price that they were new 18 months ago, or 55-60% of what good ones tend to go for now. I'm
very pleased!
PS The case is okay, but I think I will probably end up buying a vertical flip type later. If I were travelling I would probably use the horizontal flipper so I could use it as a stand to watch videos on the phone in landscape mode. It is much easier to hold the phone with a vertical case though - the screen cover just hangs down below the phone when it is used in portrait mode. That is the type that I have on my old phone and I like that design.