I am not experienced with canaries, but my understanding is that they are territorial. In your situation, I would consider getting a separate cage before adding more birds. It is likely you would need to house additional canaries in their own space to avoid aggression and stress, so if you can't do that, it might be better to not risk it.
Trios tend to be problematic, even among non-territorial birds, because two birds will pair off and the third can get excluded.
That being said, if you do add a third, it should definitely be female. The male is much more likely to accept another hen. Two male canaries in the same cage will fight.
