The sets A={1}, B={1,2,3} & C={2,4} provide a counter-example to #1 & #3.
There is a typo in #2.
I got a couple of proofs not really sure sure the best way to prove them
AcBorAcC <=> Ac(B(intersection)C)
and
AcCandBcD => (AxB)c(Axc)
and
AU(B/C)=(AuB)\(AuC)
i'm pretty sure all 3 of these statements are false just don't know how to prove it. Just to clarify u is union and c is belongs too and (intersection) is of course intersection