Most often you see cables done in stockinette st and set off by reverse stockinette on either side. If you're doing this in garter st with only a 3 st cable, it may be difficult to pick out the cable against the background. Try doing it on some scrap yarn with 3 sts reverse stockinette st, 4sts stockinette and 3 sts rev st st. Do the cable on the 4 stockinette sts and I think you'll see the cable twist.
One other thing. usually the cable is made by putting sts on the cable needle and holding to the front each time you do the twist row. Or you can hold the sts on the cable needle to the back each time you do the twist over the length of the cable. Don't mix holding to the front with holding to the back. That makes a rather unconvential cable which may not be what you want.