What is this communication standard I'm seeing? It looks like UART but with a third line
It looks like UART with hardware flow control.

Page 15 in the data sheet states that it supports hardware flow control on the USART https://www.silabs.com/documents/public/data-sheets/efr32mg12-datasheet.pdf

By searching for “uart hardware flow control” images, several plots with similar data and “ticks” appear.