No. There is a difference between an employment contract and the TN Application Letter. You will create a TN Application Letter to explain to the Immigration Officer how you and the job meet the NAFTA requirements. This will have the 3 year term specified. At the end of the 3 years, if your employer wants to keep you on, you will create another TN Application Letter and request another 3 year TN Visa. Your employment contract will not change. Use the TN Visa Expet Kit to create your Application Letter correctly.