That answer should depend on how your phone is connecting to the system. Sync 3 is still intact, so if your phone is connected by bluetooth to sync 3, or usb wired w/android auto or carplay, then you would be using the same mic setup you currently do. I remember @Guy said something about this in this thread or the Gen 3 thread.
On the other hand, if you bluetooth connect your phone to the tablet itself (and I would imagine if you used android auto or carplay to the tablet) then you're going to be using the tablet's mic. I believe in @Guy's post above, he was referring to using carplay through the tablet, not through sync 3, thus you potentially have quirks like the echo that you wouldn't have going through sync 3 directly.
What I'm a little unsure about is how the system reacts when a phone call comes in. If I bluetooth my phone to sync 3, but the tablet's focus is elsewhere (say watching youtube videos full screen) with the focus immediately jump to sync 3 for my phonecall, then jump back when the call is over?
Kind of an aside, but I do think the fact that you could connect one phone to sync 3, and a second phone to the tablet itself, is rather interesting. I carry both a personal phone and a work phone and it would be great to have both connected at the same time. Currently, personal is connected, and receiving a work call means I have to manually turn down the radio and can't utilize hands-free. With this, it shoulldn't be an issue. It would also be useful if you have a spouse or SO who often rides. I think the PX6 can actually support 5 bluetooth connections at a time, so maybe the entire family can use hands-free. Of course, my teenagers don't want their conversations on the speaker, and I don't want to hear it.
Thanks for the info.
Now another thought, what if you got the wifi hot spot for the truck activated? Say connect the PX6 to the wifi hot spot and than BT on the Sync3 side for basically phone only. I'm going to be ordering one in the next couple of days so hopefully I get the new Sync3 cable with it.