Didn't mail in my taxes last year. If I send them in this year can I still get them?

Be sure to use 2018 software for your 2018 return.  Mail it in.  If it shows you get a refund, they will send you the refund.  If you owe something, there will be interest and penalty to pay.  

Yes, you have three years from the due date of the return to file and get a refund.

Be sure to take the advice above and file using the 2018 tax software.

If you prepared them last year and just never mailed them, go ahead and mail them in as soon as possible.