Boss RC-3 Mode D'emploi page 18

Connecting to an External Footswitch
Connect your foot switch to the STOP/MEMORY SHIFT jack as shown in the illustration, and set its POLARITY switch.
When Connecting an FS-5U
Cable:
1/4" phone type
fg
1/4" phone type
16
Cable:
Stereo 1/4" phone type
fg
1/4" phone type x 2
When Connecting Two FS-5Us
or
POLARITY switch
Cable:
Stereo 1/4" phone type
fg
Stereo 1/4" phone type
When Connecting an FS-6
or
MODE/POLARITY switch