iOS6 update to 8 or greater I have an iPhone with iOS 6.0.1. I need to update it to iOS 8 or later. I also have a MacBook Pro with macOS Sierra v 10.12.5.

Problem

My MacBook has Thunderbolt 3 ports, and no usb ports. My old iPhone only has a usb cable. (I don't have a Thunderbolt 3 to usb adapter).

Question

Is it possible to upgrade the firmware on the old iPhone given my situation?

I have tried via bluetooth, I can get the devices to pair, but then don't see the iPhone in my MacBooks iTunes.

Any help appreciated.
You can't.
Due to the limitations of the iPhone 3GS and the design of Apple's iOS, you are unable to update any iPhone 3GS past iOS 6.1.6.
The only way you will be able to test you app is using the iOS simulator on your computer, or using a different iPhone.
In addition, it appears that you will no longer be able to upload 32-bit apps to the App Store, and the 3GS is a 32-bit device. Sorry!