I don't think a quadcopter is controllable with 3 engines. However, a drone designed to carry pax can be designed with redundancy - the Volocopter, for example

I don't think a quadcopter is controllable with 3 engines. However, a drone designed to carry pax can be designed with redundancy - the Volocopter, for example, has 18 rotors and engines, and can withstand at least 3 failing.