Identification of nylon 3-pin connector
This looks to me a lot like a "Molex connector", used to supply power to hard drives in older computers. EXCEPT, those connectors were 4 pins, not 3.

Going to Wikipedia https://en.wikipedia.org/wiki/Molex_connector , it seems that these are now most often not Molex brand, but "AMP MATE-N-LOK" connectors (which is slightly further confused since AMP is now called TE Connectivity.)

Wikipedia calls the specific 4-pin connector I'm familiar with an "AMP MATE-N-LOK 1-480424-0 Power connector".

The closest match I was able to find for your 3-pin connector with square keys in two corners is this: 

https://www.digikey.com/product-detail/en/te-connectivity-amp-connectors/1-480303-0/A1429-ND/19287 

It appears to be called an AMP 1-480303-0 connector. The pitch is 0.200", not 0.250". Hopefully it will match when you check yours! If not, I'd bet yours is some other AMP MATE-N-LOK connector I wasn't able to find; perhaps with that information you'll be able to locate it.