I lost power on a circuit in my house. I traced he problem to a bad light switch in a closet. When swapping it out I found that the box has 3 black wires. The switch is the start of the circuit. Whenever I hook the wire that powers the circuit my tester indicates Hot/ neutral reversed.which 2 black wires do I hook to the switch?

There is no neutral on a switch. Test hot to ground. Only one wire should be hot. The other 2 wires must be fed from the other side of the switch. Makes no difference where the hot wire goes on a single pole switch although it is usually on the bottom. If you are testing an outlet that shows hot neutral reverse, you have hot on the white side.

It worked thanks