If you've paid RCM in advance in the same month then first you may claim ITC. Yes, if you're availing ITC against RCM paid then must mention in the GSTR-3 (i.e. monthly return).
For Ex.-: you purchased some service from non-taxable teritorry in the month of july (for instance USD 10000 @ 66 = INR 660000) and you paid RCM suppose @ 5% IGST on 31st july. Now you can claim the RCM in the next month of which GST is payable of July month. Suppose other GST Payable is Rs. 90000 but you have to pay only Rs. 57000(90000-33000RCM)
Now first file GSTR-1 for outward services of which GST is payable to Rs. 90000/- on or before 10th of Aug'2017.
Then then Check GSTR-2 if there is any mismatch in ITC - this is for purchase of services on or before 15th of Aug'2017.
Now file GSTR-3 on or before 20th of Aug'2017 where you can claim RCM which you paid on 31st July
2017.
Suppose in GSTR-1 total GST payable was Rs. 90000/-
Now in GSTR - 2 Total GST ITC (without RCM) Rs. 15000/-
in GSTR-3 GST ag. output Service Rs. 90000/-
in GSTR-3 Less-: Other Input Tax Credit Rs. 15000/-
in GSTR-3 Less-: RCM Paid Rs. 33000/-
in GSTR-3 Less:- Other GST Paid Rs. 42000/-
now Closing Balance/Carry Forward Rs. 0.00