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Hello!
ArcGIS is a powerful software that oriented on GIS, Spatial data modelling, GIS data processing, GIS Data Analysis and Visualization
PCI Geomatica is a geospatial image processing suite that allows for image analysis, imagery manipulating and information extraction from imagery
Erdas Imagine is a powerful remotesensing and photogrammetry software that includes the feaatures from PCI geomatica and more
So ArcGIS is more oriented on GIS and have some tools for RS (the powerful GIS tool in the world)
PCI Goematica and Erdas Imagine are oriented on RS and photogrammetry and have some GIS tools (but Erdas is most powerful but also most expensive)
Hope that my answer will help you to understand the differents between them.