Which element in Period 3 is the most active nonmetal?
Chlorine. The most active nonmetal in any period is second to last (halogens), right before the noble gases.
Atoms tend to give electron (metals) or take them (nonmetals) to achieve a stable configuration such as a noble gas (like argon) already has. If taking just one electron makes the atom's electron configuration like a noble gas, you have an active nonmetal. That's what elements right before the noble gases like chlorine, called halogens (Greek for salt-formers) do.