Itc on credit notes

You will reduce your output liability while filing GSTR 3B instead of claiming as ITC. 

Reduce your Sale amount & GST output amount with that of Credit Note. 

While filing Form GSTR 1 mention the Credit Note in Credit Note/Debit Note section.  

Credit note received in relation to service procured basically reduce your ITC, hence you have to reduce you ITC availed on the original invoice.