I would not be as optimistic as Bimpelrekkie. The moisture will be an issue, but temperature may be one as well. For cost reduction purpose, the Raspberry Pi may use consumer grade electronics, which is usually rated for 0 to 60°C. Automotive grade electronics are expected to go from -40°C to +85°C (ambient) at least (some parts shall work under higher temperatures).
On a RPi3, the CPU is a Cortex-A53 designed for low power consumption and should not overheat first. I don't know the GPU. If the SoC does not fail first, predictable issues may be:
- Insufficient power regulation as capacitors characteristics changes with temperature
- RAM corruption due to overheat and insufficient refresh
- RAM corruption due impedance mis-match (unlikely, as DDR3 performs Zq calibration within the protocol)
- Flash memory retention issue
Also, the RPi may fail due to vibrations.
Finally, it may also work perfectly fine. I'm just stating that it's not granted, and I'd also like to add that it may vary with the RPi production process. A prototype may work for days, weeks, maybe even months (it may also fail after a few hours), but you can't industrialize anything with a good relation with your manufacturers and a good knowledge of the components you're using.