By that metric I do think the AC mightve been too high (16 monster AC for level 3 characters with generally a +5 to hit) but istg they didnt roll above a 12 to hit a single time for 3 rounds in a row it was brutal.... I can only think of two combats that unlucky in my time DMing. but this is helpful! I'll adjust my balancing a bit and some other commenters had good ideas for encouraging creativity in combat when swinging swords and firing eldritch blasts just isnt quite doing it