Begin with a player out wide with the ball, a player in the middle and a player on the far side. There is one keeper in goal and an opponent marking the far player.
The defender starts by overplaying one of the runs to make it easier for the player with the ball to read the situation but quickly, this changes to the defender being live and the player on the ball having to make the correct read each time. Next, add a second defender.
The player with the ball can now either play the ball to the diagonal runner, play the ball to the looping runner or, if the defenders take away those two runs, can dribble to goal and shoot.
The key to this activity is it's not a 3 v 2 scrimmage but rather, a given situation being recreated so players can learn to read space, read runs and make appropriate passes.
It's important for all players to get the opportunity to play each of the roles because not only might they find themselves in the position to make certain runs, they also will understand how to defend them if they understand the mentality of the attackers.