The 3rd and final player to arrive at the ruck is called the “Firewall”. The role of the Firewall is twofold. Firstly, to ensure the ball is inside the hindmost foot of the ruck. The Firewall ensures this is the case by binding onto the Combat and ensuring they are binding over the ball. The Firewall Binds on the Combat from behind the Combat thus increasing the length of the ruck and ensuring the ball remains in the ruck until the scrum-half is ready to pass it away.
Secondly, the Firewall must stabilize the ruck when binding onto to the Combat. The Firewall can achieve this stability by binding onto the Combat in a strong body position that in essence resembles the body position of the Combat. As with the Combat, the Firewall must retain that strong body position throughout the duration of the ruck. If the Firewall gets into a weak body position and the defenders counter-ruck the Combat and Firewall can be driven off the ball. Or at best the looseness of the ruck can cause the ball to pop out from under the Firewall, which means the ball is out of the ruck and the ruck is technically over.
Once the Firewall binds onto the Combat, with the ball under the Firewall’s feet and they both retain a strong body positions, the ruck is now formed. This ensures the ruck is structured in the optimum way using just three players (Cleanout, Combat & Firewall) along with the tackled play who is quiet often still on the ground. This formation, if executed quickly and with good technique, should ensure quick ruck possession.
As a further insurance against the counter-ruck the Cleanout can contribute in a secondary role. Once the Cleanout removes the Threat their job is effectively completed as the Cleanout. The Cleanout is often on the gound after executing the Cleanout. In this situation the Cleanout can get to their feet quickly and re-enter the ruck through the Gate and become a second Firewall.
They do so by binding onto the side of first Firewall. This stabalizes the Ruck even further and pretty much ensures the ruck will be resistant to any form of counter-rucking. On this basis it should be possible to win any ruck using just 3 players along with the ball carrier (a total of 4 players) in most situations. This will free up the other 11 players to launch the next attack.
But if any of the players at the ruck, including the tackled player, fail to execute their role efficiently, it will more than likely require another player or players to get to the ruck to ensure the possession is retained. Any extra player involved in the ruck results in less attackers available to launch the next attack.
The other possible occasion when more than 3 players may be required to win a ruck is when a player is caught in possession well behind the gain-line, e.g. going back to collect a kick in the backfield or getting tackled during a counter-attack from deep. In this situation it is more likely a case of retaining possession regardless of the number of players needed. Most teams usually kick possession from this situation.
If a team can consistently present the ball well in the tackle and get 3 players to the ruck quickly and all there players execute the roles of Cleanout, Combat and Firerwall efficiently, quick ruck ball is a very reasonable expectation.