Arsenic is an element that reacts with many things, which I am not going to list because it is impossibly long and complicated. Look it up.
Tricalcium diarsenide is the result of reaction: Ca3As2.
Yes: reactions with HBr, HI, H2S, H2Se, H2Te
yes
It will create arsenic acid, nitrogen dioxide and nitrogen monoxide 3HNO3 + As --> H3AsO4 + NO + 2NO2 + Heat