If you look at the three materials, they are all of an extremely large size and only have covalent bonds. So they are all giant covalent structures. As there are a lot of covalent bonds bonding atoms together in a tight network, these would have extremely large melting point, and would be very rigid.
Noted - graphite is rigid in the sense that you cannot easily crush it. We can use graphite to write because graphite atoms are in a huge number of slides and each of them can slide past each other.