Here's a look at how the 3 metros have grown since 2006:
Data Source
Overall during this period, greater Moncton grew by around 22k, Fredericton by 15.5k and Saint John by just under 4k with a few years of decline between 2011 and 2015. In percentage terms, Moncton and Fredericton had very similar growth rates (16.9% and 16.7% respectively) from 2006-2018, with Saint John coming in with a rate of 3.1%.
Averaging the rates above to annual growth rates, I extrapolated the CMA/CA populations out over the next 20 years:
So if we assume the same long-term growth rates that we've seen over the past dozen years, greater Fredericton passes greater Saint John around 2035.
Of course, a lot can change in that amount of time