+3 full plate, +3 shield, ring of protection, defense style = ac 28, i am assuming not maxed out hence 24-26
The problem is that CR doesn't assume
any magic items, let alone legendary armor or shield, let alone legendary armor AND a legendary shield. I mean, a fighter with that kind of bling should be a match for a balor. Which is not to say that it is worthless if you give your characters magic items -- there's some play there. Merely that it's not designed with the assumption that characters will be blinged out like that.
Also, was your calculation taking into account the balor's fire aura? Because it seems to me that, even with the AC you've chosen, in a slug out the fighter should be taking an extra 32 points of damage (10 + 7.5 + 7.5 + 7.5, rounded down) per round. 62 damage per round should drop the fighter in 5 rounds. The fighter does 31 points of damage per round, which means it takes him 9 rounds to kill the Balor. Actually, more rounds. I calculate the fighter's damage as +1 longsword (average damage 5) + 5 (damage bonus) plus superiority die (average damage 6), for 16 points of damage, but only for six attacks (two rounds). At 70% chance to hit, that's 11 points per attack, so 33 points for the first two rounds (66 total), at which point he runs out of superiority dice. After that he's only doing an average of 10 points per attack -- three attacks at 7 average damage is 21 points per round. That's another 10 rounds, for 12 total. We can go ahead and give him a +3 longsword, but it's still 10 rounds total. He's going to need the additive effects of at least three mates to spread out the damage and bring that number down to 4 or 5 rounds.
Of course, if we go with the no magic item assumption of CR, the fighter's looking at an AC of 21 (plate, shield, protection style), which means the balor's hitting 70% of the time, so the balor's doing 41 + 32 points of damage per round, and dropping the fighter in 4 rounds, while the fighter is now only doing half damage per round (resistance to non-magical weapons), and it takes him over 20 rounds to drop the balor.
I dunno. It sounds to me like CR
as a baseline is working fine. Some groups with lots of magic items and/or synergistic teamwork are going to have a relatively easy time. Others without so much magic or optimized teamwork are going to have a bit more trouble. A lower level group with 6 characters is going to have a much easier time. Sounds like a Medium to me.