It doesn't say if it's a right or left cable, so I guess you do whichever you want. Cable 3 is doing a cable with 2/1 instead of half and half.
C3L - Cable 3 Left. Slip next st onto cable needle and hold at front of work,knit next 2 sts from left-hand needle,then knit st from cable needle.
C3R - Cable 3 Right. Slip next 2 sts onto cable needle and hold at back needle, then knit sts from cable needle.