This response is based on the information provided. We suggest you discuss with your finance team, contracting team, program manager and/or legal department as appropriate.
You find fiscal law rules in the Financial Management Regulation (FMR), the document one might call the FAR of finance. If you have FY 18 3-year procurement money, it is useful for obligation until 30 SEP 20. It must be used for its original purpose which can be fairly broad. The effort must be within the original scope that the money was appropriated so you need to refer back to the budget exhibits originally submitted. You also need to ensure that the contractor has submitted all of the invoices for this contract.