At its best:
Turn the container upside down and drill four or five drainage holes in the bottom.
Add compost until it's two-thirds full.
Plant the carex in one corner of the cube pot. This will form the 'back' of the finished display.
Fill the remaining corners with the three hellebores, and push more compost into any gaps.
Water the plants in well and stand in a sheltered spot. Make sure the compost doesn't dry out in winter winds.