Genuine Deans brand 3-pin battery connector. Highest quality connector, for small-to-mid size amperage motor applications. Original Deans part # 1003.

The battery end (the one that is ON a battery pack) only has pins sticking out of 1 side.

Polarity of pins  (see picture, going Top to Bottom):

Black (NEG-)

Red(POS+) center

Signal (or just not used).