I want to re use repurpose an exterior hvac fan. There are three wire from the fan motor. One gray, black and blue. How can I wire this fan to a regular cord to plug into a receptacle? I understand the need to have a cage etc around the blades...

What is the voltage?

There must be a tag or engraving somewhere on the motor, what is it?

It's very possible that your fan either is not 120 volts or needs a capacitor to operate, or both. Without the mfg. information it will be impossible to direct you to the next step.