Can I claim unused medical/dental receipts from several years ago? For example, are unused medical expenses from 2014 to 2016 allowed to be claimed on my 2018 tax return?

For 2018, you can only claim medical expenses for a one year period that must end in 2018. So you may be able to claim some 2017 expenses on your 2018 return, but not from previous years.

You can go back to your previous years returns and add the expenses from those years, through the CRA or through ReFILE: https://turbotax.community.intuit.ca/replies/2647992