If I reroll 3 high tier champion skins will it give me a better skin?

random.

[{quoted}](name=Hai i fix ur hat,realm=NA,application-id=cIfEodbz,discussion-id=PrbzdcZb,comment-id=,timestamp=2016-12-08T21:30:29.318+0000)

Or it it all just random

Assuming I read the title correctly, yes, it is correlative to the shards you sacrifice.

e.g : Re-roll three skin shards each worth 975 RP and you'll get a random skin worth the same 975. The same applies to sacrificing three 1350 skin shards as well as 1820.