If you're wondering about the performance of hashing [i16; 3] versus i64, you're seriously thinking too much about this. Both are going to shove a bunch of bytes into a hasher. Maybe the array is faster because it's fewer bytes. Maybe i64 is faster because... I dunno, something something alignment. Who knows?
Profile your program first to find out whether this hashing is actually performance critical. And if it is, then benchmark each of the alternatives.
You'll probably get a much bigger speedup by swapping the hasher out with e.g. FNV.