Euphobia characias subs. Wulfenii is easy to grow since it will tolerate varied soils so long as its free draining, doesn't mind being exposed and will happily get on in some shade.
Saying that, I had one growing in really poor soil in my front garden that got shaded out by a large ceanothus and was looking extremely straggly so I chopped it back low and moved it to a semi shade border with slightly deeper soil in the back garden and the thing was instantly away. It's put on staggering growth in the last three years.
So, from my experience, it could be that there is some difference between the good one and the two that are unhappy. What this is, is difficult to assess without knowing your situation, but my novice guess would be too much shade or water. Study the movement of the sun or soil condition in their location over a period of time perhaps, alternatively, just move them anyhow. If I understand it to be the Euphorbia you have stated, it will grow very large and sprawling with time and unless you have a huge stately home, three together might all get a bit forestry.
Repetition in the garden is good, so personally, I would dig up the two sickly ones, inspect the roots and replant in desirable positions to show them off at their best. Good luck
