Like the poster above me, I also have a Pi 3B+ with a 7 inch touch screen, V1.1 from PRC. I don't use the GPIO pins for power. Instead, I use a 5V/2A USB adapter, so I don't have the burning smell issue.
(The poster above me may need to run an adapter for the Pi 3B+ with more amps, 2.5A at least.)
Is there some kind of firmware update needed for the touch screen? I tested this with two micro SD cards I specifically run for the 3B+. Each time, I got nothing but a black screen. I can get to the console only, minimal version of Raspbian on both cards via the 7 inch screen.
I'm typing this right now on the Pi 3B+ with a HDMI TV, using the card with the experimental install (not that it matters, but anyway...).
To add: I have the original Pi 3, and the 7 inch touch screen works there without any issues.