I was replacing outlets and switches in my living room. The last switch I came to was for an outside light. When I removed the switch plate there were 3 black wires, no white or ground. Two of the wires plugged into the back of the switch and the other attached to a side screw. I attached one wire to the top side screw on the new switch and the other two to the other side screw. Now the outside light is on all the time and the switch turns the two outlets in the room on or off. Whats up?
Sounds like you removed a 3 way switch and replaced it with a single pole switch.
If the old switch does not have on/off marked in the handle and it has three terminals it is a three way switch. You need to replace it with the same kind.
One of the wires is either the hot feed from the panel or it runs to the light. The other two wires are travelers running to another switch somewhere.