Replacing switch: three black wires???

Hi, I've been replacing all the old outlets and switches in my '70s house for aesthetics. I got to the last single-pole replacement and I'm baffled.
I found three black wires attached to the old switch: two punched in holes in the top, one in the bottom. The box is metal and inside there are pig-tailed white wires as well as twisted copper wires, but they aren't attached to the switch.
This single-pole switch only turns on a light above the shower. I suspect the extra wire may power an outlet in the hall outside the bathroom. A Lowe's employee hinted the extra wire is likely a traveler, but wouldn't tell me anything else because of liability.
I'm trying to replace it with a simple single-pole switch. Any advice is much appreciated.

 

Welcome to the forums.

That is not a three way switch. You know how I can tell ?
A three way switch doesn't have OFF and ON on the handle.

Refer to my generic switch picture.
The two wires in the black circle are both hot. Instead of connecting them together with a short tail.... they used the lazy mans connection method..... screw terminal and push in. Both of those wires are connected inside the switch.
The wire in the red circle is the other terminal.

 

Thanks for your quick response.
I know about the "on" and "off." I know it's not a three-pole switch. Are you saying it appears to be three wires, but if I pull them out of the switch, I will actually only have two wires? And each wire should just be attached to the screws on the new switch and I'm done?
Ah, man, if I find out I did all this fretting for TWO black wires...! Thanks!

 

No. See diagram below: .

 

Most helpful! Thanks very much!