I cant imagine how else its suppose to work ...
You have 3 enemies ... two in Leather Armor, one of them with shield ... one in Chainmail armor with shield ...
You attack an enemy ...
You have no idea if the strike will land or not, since you dont have any estimated %, or info about their AC ...
And now what?
How would you know if you missed, just bcs you rolled 2, or bcs their AC is 25 ? O_o
Based on what would you decide if you want to try your luck with another enemy, or try the same one again?
The way it usually works, in my experience, is trial and error. You can see your rolls and know which ones hit and which ones didn't. So you try a few different things, prodding your enemy's defenses, to figure out "how difficult is this guy to hit?" and then adjust your strategy based on what you learn. If you make a bunch of attacks that feel good (high rolls) and you're still missing, you adjust your strategy.
There's no reason, upon seeing someone that you've never encountered before, that you would immediately know the best way to fight them. I could see spending some time to study an enemy before a fight to roll a perception or insight check to get some clues.
Actual numbers for your example:
Leather armor is AC 11 + DEX mod. If leather armor gives you an AC of 19, your DEX would have to be at least 26. Anything over 20 DEX is very difficult to attain.
Same goes with the Leather+Shield guy.
Chain Mail is AC 16, where leather armor typically maxes out.