This should work:
/script if not buffed("Corruption", 'target') then CastSpellByName("Corruption") end;
/script if not buffed("Curse of Agony", 'target') then CastSpellByName("Curse of Agony") end;
/script if not buffed("Immolate)", 'target') then CastSpellByName("Immolate") end;