Level = mixer channel
The other two are somewhat paired. The sample is the lowest of the level settings and the volume is sound which contains a sample. Different settings are made for gain staging which allowing you to get an overall uniform level.
If you are only using 1 sample per track and not doing any sample flipping, it doesn’t really matter what one you adjust. The end result will always be the same.
When you start getting into flipping to different samples/sounds on the same track, you need to be careful that the volume of each one is similar so the volume doesn’t jump over the place.
Sample flip - adjust at the sample level to match.
Sound flip - adjust at volume of each sound to match.
Mixer level would be best suited for mix/performance.