Quote from P.11
4.1 "Units are Shaken once they have received a number of Suppression Marker equal to the number of current models in the unit."
4.1.1 "Check to see if a unit is Shaken after it receives any Suppression Markers (either from an Assault, Close Combat or Shoot Action). You should completely resolve the attacking unit’s action before checking to see if the target unit breaks.
Do not add any additional Suppression Markers on a unit after it is Shaken, you should also mark it in some way to show that it is Shaken."
4.1.3 "Shaken units may only have as many Suppression Markers as models (or HPs); they do not receive any additional Suppression Marker after they have been shaken and before they regroup."
So in your example the unit becomes Shaken, the surplus markers are not placed.