Another approach might be to have the top and bottom labels have constraints relative to the view top and bottom, respectively, and have the middle view have top and bottom constraints relative to the first and third view, respectively.
Note that you have more control over constraints than it might seem by dragging views close to one another until guiding dashed lines appear - these indicate constraints between the two objects that will be formed instead of between the object and the superview.
In this case you would then want to alter the constraints to be "Greater than or equal to" the desired value, instead of "equal to" to allow them to resize. Not sure if this will do exactly what you want.