Hi Kai! These don't all mean the same thing - they are three different tenses. 1. "I never saw snow" is past (e.g. I never saw snow when I was younger) 2. "I never see snow" is present (e.g. I never see snow now) 3. "I've never seen snow" is a tense called the perfect. (e.g. I've never seen snow before). This tense is pretty difficult to explain so I recommend looking online for videos etc. that explain it. Although the sentences have different meanings, its possible that you've heard some natives use them incorrectly.

1. I never saw snow, is in a past form 2.I never see snow, is in a present form 3.I've never seen snow, I have never seen snow it means in your life you have not saw any snow at all I hope i helped at least a little