More cannon rules. Multiwound on multiwound unit

• BRB p. 45: If a unit of creatures with more than 1 Wound on their profile is hit by a weapon that causes multiple wounds, determine how many wounds are caused on each model individually (remember that each model cannot suffer more wounds than it has on its profile). Add up all wounds caused on the unit and then remove the appropriate number of models, noting any spare wounds on the unit.
  
  This method is also applied if such a unit is attacked by a spell or weapon that causes a hit on every model in the unit.

In the case of cannons, there is an exception to this rule regarding monsters or monstrous units: a model must be killed before the cannon ball can proceed to the next rank. So, in the case of the Ogers, each model must suffer three wounds. The rules do not spell out what to do with already existing wounds, but the most logical way to handle that is in my view to attribute the floating wounds to the first model hit (and remember that excess wounds do not carry over).