The network-forming liquids (SiO2, GeO2, Al2,O3) were studied using molecular dynamics simulation at 3200 K and in a wide range of pressures (from 0 to 25 GPa for SiO2
0 - 48 GPa for GeO2
0 - 20 GPa for Al2,O3,). The results show that their structures are of three structural phases: TO4, TO5 and TO6, (with T = Si, Ge or Al). For SiO2, and GeO2, systems, the TO5 phase forms mobile regions. Meanwhile, for Al2,O3, system, AlO6 phase torms mobile regions. By visualizing molecular dynamics data, this paper clarifies structures and properties of materials at atomic level, namely results showing the structural phases, the phase transformation, atomic clusters and mobile regions in these liquids. In addition, this visualization reveals the existences of polyamorphism (structural heterogeneity) and origin of spatially heterogeneous dynamics in network-forming liquids.