Perhaps the 3 bags were actually oversize for cabin rules and the AC agent was correct in checking them?
...reference to the size or number of bags...
There were four of us travelling, and we each had one carry-on bag that were similar in dimensions and were within AC's carry-on size dimensions as well. Three of them were forced gate-checked, and the fourth one went into the cabin with us and it fit in the overhead bin without any problems!
Airlines are starting to go crazy with this, believing that it will save time boarding if they can force gate checks without bothering to verify whether there really is bin space or not.
The flight was already late coming in from its previous flight, so this could probably be why the agents stopped people to check-in their carry-ons to try and speed up boarding. Our flight was still late departing YYC and arriving into YVR.