The ordered and disordered phases of three unsymmetrical cyclic nitramines: (I), 1,3‐dinitro‐1,3‐diazacyclohexane; (II), 1,3‐dinitro‐1,3‐diazacycloheptane; (III), 1,5‐dinitro‐3‐nitroso‐1,3,5‐triazacycloheptane