How long do you have to amend a tax return? I sent in an amendment for 2015, the taxes filed in 2016. Mine was denied:( it was sent after 4/15, but I was told it was ok

I just received a denial saying I was after the 3 years. But I was told for an additional refund the April 15 didn’t matter. I sent in the amendment after I got my regular taxes completed as I though I had time. I sent it a few weeks after 4/14:(:(. Is it worth appealing. I have the screen shot of the information from turbo tax saying for an amendment I didn’t have to meet the 4/15 date.