I would like to generate hdfs disk usage report by folder wise for past 3 months.
A report like this is probably best generated with SmartSense 1.3. Take a look at the example dashboard:
Take a look at the documentation here: http://docs.hortonworks.com/HDPDocuments/SS1/SmartSense-1.3.0/bk_user-guide/content/ch_smartsense_us...
The specific report you are looking for may not be there, but you may be able to modify the dashboards to generate one.