The spreaders would just flip up and follow the load into the trench
Perhaps an extreme example of the point. However, the underlying consideration remains valid - shoring stiffness.
The area of the load is 1.5x3 or 4.5 m2. 2000/4.5= 444 kg/m2
'fraid not. The floor knows naught of the box as it isn't touching the floor. All the floor knows about are the two shoring planks .. which are. Providing that the shoring stiffness is adequate to spread the load the floor only is aware of the shoring planks' loading stress. ie 2000/3 or 667 kg/m2. By a process of deduction, the floor then figures that the load is 3 x 667 ie 2 tonne.
If you used 667 kg/m2 ..
'fraid not. Respectfully suggest you revise Mechanics Lesson One.
Looking again at the sketch again, running load is kg/m rather than kg/m2. Missed the error first time around ..