Finding stress in three rods

Looks like the outside columns equally support the portion of the load up to the point where their deflections equal 0.3 mm. Calculate what that load is for that deflection, and the stress in the outside columns (no stress or deflection in the interior column) under that load. Then the remainder of the load will be divided equally amongst the 3 columns.

 

Another way of doing it is to let the displacement of the top cap be x downwards.

Then assuming all the columns take some load, the strain in the end posts is x/125 and the strain in the middle post is (x-0.3)/125. Find the force in each column corresponding to the strain, and get an equation for x by saying the forces in the 3 columns sum to the total load.

If x comes out as less than 0.3, then the middle column did not take any load, in which case finding the load in the two end columns is easy!