I came across the complete pin mappings for this 3.5" RPi TFT display buried in the aliexpress site. Just in case anyone needs them:
Interface Definition:
PIN NO. SYMBOL DESCRIPTION
1, 17 3.3V Power positive (3.3V power input)
2, 4 5V Power positive (5V power input)
3, 5, 7, 8, 10, 12, 13, 15, 16 NC NC
6, 9, 14, 20, 25 GND Ground
11 TP_IRQ Touch Panel interrupt, low level while
the Touch Panel detects touching
18 LCD_RS Instruction/Data Register selection
19 LCD_SI / TP_SI SPI data input of LCD/Touch Panel
21 TP_SO SPI data output of Touch Panel
22 RST Reset
23 LCD_SCK / TP_SCK SPI clock of LCD/Touch Panel
24 LCD_CS LCD chip selection, low active
26 TP_CS Touch Panel chip selection, low active