Your 2018 tax refund will not take care of the 2015 amendment liability because those tax returns will be processed separately and within two different timeframes. It takes about 16 weeks (sometimes longer) from the day the IRS receives your amended return for it to be processed. Your 2018 tax refund will come much sooner.
I recommend that you file 2018 now and when the refund comes, file 2015 amendment with the payment.