I am a 3rd year student of BBA & want to do MBA & Ph.D after it in the same college. Which is the best college for the same in Kolkata?

MBA and PhD in the same college is a rare possibility. Please do not constrict yourself to a single city. MBA is all about exposure and practical learning, it's always preferable to move out to different state or city. You have AMmity University in Delhi which provides such course and also ISB Hyder

You may consider Indian Institute of Management, Calcutta and Department of Business Management, University of Calcutta as these are two best options for you to do MBA and PhD from same college.