Weak hits only happen if you are weak affinity - IE, for example, you're blue and attacking red.
Likewise, strong hits only happen against strong affinity - you're blue and attacking green.
Critical hits are independent of the above. The calculus, as far as I'm aware, is to first check if the hit is strong or weak. Weak hits can never be critical. When you're weak affinity, your crit rate is also capped at 85%. So, I believe if you're weak affinity and have 85% crit rate, your crit chance will become:
(1 - 0.3) = 0.7 * 0.85 = 60%.
Or, put another way, 30% of your hits would be weak hits, 60% would be crits, and 10% would be normal hits.