Yup, three points determine a plane, but gravity is acting on this four-cornered car directly perpendicular to the plane…
Isetta thought three points would be stable on their original 3-wheeled design… but they kept falling over going around corners, so they placed two rear wheels 6" apart to solve the problem (true story).
The bottom line is that I would not risk my life or anyone else’s based on the proposition that three points define a plane… for anyone reading this, I strongly urge you not to get under a vehicle on three jackstands.