I believe 2 carriages per train is an optimal design for Canada Line since cost effectiveness plays a major role in this regard. Many of the constraints come from the Cambie corridor... And, in the big plan of GVRD, Richmond is always not involved in the agenda for many reasons... Southwest greater Vancouver is not a proper area for high density population.
Not only more, but way more... And, the worst part is, the line may not be built in time... City of Vancouver has a bigger control of the budget. One of the priority is: The line has to be built on time...
It also depends on the schedule as well... At peak hours, when train arrives at +/- 6 to 8 minute intervals, there could have been jams at the entrance.
But...
It doesn't count in the case that if one breaks down? I believe they may need one more each way at Richmond Center... Aberdeen and Lansdowne should be okay with 3.