Groove the third man combination as a three player unit: the pass in, the first time set, and above all the run that starts before the first pass arrives.
Three cones in a triangle, the base about 12 yards behind two forward points that sit roughly 11 yards apart. One player on each, one ball at the base. Play up into a forward point. He sets it back first time without turning. As that first pass is travelling, the other forward point starts his run inside. The base plays the through ball into that run, first time if possible. The runner takes it in stride, carries out, and returns it to the base. Mirror the next rep so the other forward point makes the run.
The third man run is the combination that beats an organised defence, because the player who receives the decisive pass is never the player the defence was watching. Two defenders track the ball into the target and the set back to the passer. The third player is the one nobody adjusted for, and by the time anyone reacts the ball is already past them.
Running it in a triangle strips the idea down to three actions and one piece of timing. The pass in has to be firm enough that the target can set it without generating pace himself. The set has to be one touch, because a controlling touch gives every defender time to reset and, in this exercise, gives the runner time to arrive too early. The through ball has to go into space.
The timing is the whole thing, and it is the part that is nearly always coached backwards. Players are told to run when the ball is set. By then the pattern is late: the passer is waiting on his heels with the ball, which in a match is exactly when a defender arrives. The run starts while the first pass is still travelling forward, so that the set and the run finish at the same moment.
Mirroring every rep matters for a reason that is easy to overlook. If the same forward point is always the target, one player spends the whole session setting balls with the same foot and the other spends it sprinting. Swapping which point runs means both players learn both jobs, and the set with the weaker foot gets exposed before a match exposes it.
When it works, the giveaway is that nobody stops. The ball goes in, comes back, and leaves again in about a second and a half, and the runner is at full speed when he takes it.
Save it to your private drills, then change the animation, adjust positions, swap equipment, or drop it into a session block. Or start from a blank pitch and build your own.