Do you only enter the payments for 2017 of lease pymt or do you enter the total cost of the least (36 months) under the "Cost Excluding Tax" field for the Sale Tax?

Bought the a leased vehicle in Aug, 2017.  Under the Cost Excluding Tax field.  Do I only enter the lease payments from Aug thru Dec, 2017 (not the sale tax) or do I add all 36 months of the price of the lease cost in the Cost Excluding Tax field?    This is under the Enter Major Purchase for the Sales Tax section under Deductions and Credits.  Thank you