soft whistle: yeah the above^ vocals or soft melodies whatever like piano for extra emphasis
normal whistle: for strange sounds like ones in tech songs to emphasise them, rarer usage these days i feel like. For old style mapping idek, it is still used alot for vocals or more prominant sounds in place of other hitsounds, old style hitsounding is hard to describe
drum whistle: i have no idea, if that sound ever pops up in a song then yeah- lol
It all comes down to the same thing as other hitsounds, when theres a sound in the song youve mapped, just try to match it with a hitsound that is similar; if there a whistle or light sounds in the song then use whistles