Various forms like Durga, Chandi, Mahakali, Mahalakshmi and Maha Saraswati are but Her divine forms. Each form of the Devi has a distinctive role meant for a definite purpose. Yet every form is also identified as the Ultimate Reality of the universe. Each form of the Divine Mother is thus a part of the Supreme Divinity. One form may appear different from the other. But in spirit, each such form is one more manifestation springing from the One
Source, verily the Supreme Primordial Prakriti.
Though the Devi Mahatmyam constitutes chapters 81-93 in the Markandeya Purana, are basically description from purana. It has an intrinsic independent status by itself. It is a full-fledged scripture by itself. Part narrative and part hymn, the Devi Mahatmyam combines the strengths of both the oral and written traditions. On one hand it is like a synthesis of many myths from many sources, skilfully integrated into a single narrative and thus incorporates the best of the puranic approach. On the other hand it also displays the bardic style of the vedic approach that combines the best of preliterate and literate strengths of expression, in so far as the hymns are concerned. Although the four hymns are integral to the narrative, they are markedly different in quality.
Because the Chandi integrates three major narratives in its portrayal of the glory of the goddess, the Chandi naturally falls into three parts corresponding to the three gunas- sattva, rajas and tamas. Thus the entire text of thirteen chapters is grouped into three sections comprising of three major episodes (caritras) each one describing the glory of one of the three different aspects of the Supreme Goddess, three Goddesses identified as Mahakali, Mahalakshmi and Mahasaraswati. Although they are represented as three distinct images, they are virtually one and worship of one form is adored as the worship of the other forms.
Ref : Sri Sri Chandi, Wikipedia