While there is no limitation if you owe taxes, the IRS has a three year statute of limitations on refunds. They will only honor refunds on an amended return for the current tax year and three years back. So for you, anything you may be owed for a 2015 return or older can be filed - but you will *NOT* be paid it. So don't waste the time.
Also, for 2016-18 if you are prior military I would "highly" suggest you visit your local VA office (right now, during tax season) for help with this. If you use TurboTax to amend a past year's return it's going to cost you money. The VA folks can help you with this for free.
Additionally, as a either an honorably discharged or medically discharged under honorable conditions, you also qualify for the services of the local "Retires Activities" office (RA Office) if there's a base or other military activity near so that has that. The RA office is very busy this time of year with qualified vets this time of year, with tax related matters. It's also free.
