* 조금 = a little.
* 약간 = a little. slightly.
They are about the same in meaning, but 조금 seems to fit wider contexts. For example, 조금 works equally well in a negative sentence but 약간 starts to sound a little unnatural: 조금도 틀리지 않는 말 (O) / 약간도 틀리지
* 살짝 = 1) lightly or furtively (said of a motion), 2) just a little.
살짝 describes the way something is done in a very light, almost unnoticeable way.
It can also mean "just a touch of something", indicating even smaller quantity or degree than 조금/약간.
In terms of quantity, I think it goes 조금 > 약간 > 살짝, with 살짝 meaning the least or the lightest.
However, 살짝 has a more limited usage than 조금 and 약간 because it sounds very colloquial and often implies movement.
- 후추를 조금 넣었다/쳤다 = I added/sprinkled a small amount of pepper.
- 후추를 약간 넣었다/쳤다 = I added/sprinkled a tiny bit of pepper.
- 후추를 살짝 넣었다/쳤다 = I added/sprinkled just a touch of pepper.