It means Everton, Aston Villa and Fulham will play their European matches with a total of five match officials, communicating via headsets.
The idea was the brainchild of UEFA president Michel Platini and he has been championing the scheme as an alternative to goal-line technology. It was tested last year in some European Under-19 championship games.
FIFA president Sepp Blatter said: "The International FA Board decided 'let's have experiments with two additional assistant referees' and we were looking where we have can such experiments. Now we have found a solution together with Michel Platini and UEFA."