I doubt it was possible to become a citizen in 2015 after PR for three years only. The waiver allowing credit for time spent in Canada before PR came when Liberals came into government in 2015. Something may not be right with that citizenship application.
That's actually not quite accurate.
The credit for pre-PR time existed before the 2014. My husband applied for citizenship in 2012 and was able to claim a year of pre-PR time.
The changes to the rules introduced in 2014 removed the ability to claim pre-PR time and also increased the residency requirements by a year.
The Liberals then reversed these changes.
In this case, it is possible that the sponsor applied for citizenship in early 2014 under the "old" rules where credit was given for pre-PR time and was approved / took the oath in 2015. So the math can in fact work.