I mostly base my party composition not around being balanced, but around story and banter. I play on casual anyways, we're all going to survive even if it's a party of 4 mages, haha.
I also prioritize story and banter, but I have to make sure to take one of each class most of the time, because there's usually something class-specific that I'd rather not have to come back to get through, like locked doors, the plethora of veilfire braziers and to a lesser extent walls to bust through, though sometimes explosive shot does the trick. If you're a mage, it's so much simpler, because the majority of class-specific stuff in this game is for them. Heck, most of the darkspawn missions in this game require a mage to resolve, since only they can seal tunnels.
I'm glad that top 3 covers all my bases (Sera, Dorian and Cassandra).