If you have a variable speed well pump, it is very likely to contain a three-phase motor. What it doesn’t have is direct wiring from the motor windings to incoming power. It likely has a variable-frequency drive consisting of an AC/DC converter (for which there would be a small but nonzero benefit if the supply were three-phase supply) and three inverter outputs connected to the windings. And hopefully either the VFD is in or very close to the motor (Grundfos will sell you a VFD/motor/pump combo like this) or has proper filtering to reduce high harmonics and properly shielded wiring to the motor.