When individuals are damaged by an illegal arrangement to restrain trade, which law allows them to pursue civil action and recover up to three times the damages sustained?

It is the "Clayton Act".

Clayton Antitrust Act which was passed in 1914 by the United States Congress as a revision to clear up and supplement the 1890 Sherman Antitrust Act. It was drafted by Henry De Lamar Clayton.  The Clayton Antitrust Act was the reason for a considerable number of vital suits against vast organizations.