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1) When two 3-way switches are connected together, only one of the 3-ways will contain the hot circuit feed that terminates to the black screw. The 2 brass screws are the traveler wires. The other 3-way switch on it's black screw will be the switched loop that extends to the lights. You will need to confirm which of the 3-ways contains the hot circuit feed. I suggest using an AC voltmeter to confirm this.
2) Once you confirm which of the 3-ways contains the hot circuit feed, you will then need to make up a 6" pigtail wire of the same wire size and splice it onto the 3-way hot feed wire and the other wire will then be your hot circuit feed into the new single pole switch. Then terminate the pigtail to the black screw terminal on the 3-way. By using a pigtail connection, you now wind up with 2 hot circuit feeds.......ie...... 1 going into the 3-way and the other going to the single pole. Both switches will now reside on the same circuit.
3) In your application, the 2 blacks are the travelers and the red wire will either be the hot circuit feed or the switched loop. The red wire needs to be confirmed using an AC voltmeter or a 2 wire lead AC voltage tester.
4) Keep in mind that the single pole switch can only originate from the 3-way switch that contains the hot circuit feed and not from both 3-way switches.
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