Riverkeeper, a non-profit watchdog group, is taking three Brooklyn companies to court over their alleged pollution of the Gowanus Canal. Riverkeeper filed intent to sue notices against Sixth Street Iron and Metal, Ferrara Brothers Building Materials and 107 Sixth Street LLC, all of which, Riverkeeper claims, dumped hazardous materials in the canal. Joshua Verleun, a Riverkeeper investigator and staff attorney, said that the action signifies his group’s vigilance. “It’s a shot across the bow,” Verleun said. “It’s a way of saying we’ve been patrolling, we caught you doing this, now you need to do something about it.”