Retirement tax questions

We filed our taxes already and are just now realizing this. Will we be able to do an amended return without an additional fee and mail in whatever we owe before April 15?

 

Also, we paid taxes when we got our distribution and now it seems like we are being taxed on the full amount spread over 3 years and not acknowledging that we have already paid in taxes. Will a TT person be able to help us with this?