My party of mixed 2nd and 3rd level PCs were stuck in a fight versus what was essentially a town guard (plus friends) with 20 AC, leading to multiple players wasting multiple turns rolling to hit and missing every turn.
Is it in any way normal to be facing that kind of AC at this level?
20 AC is unusual, but not unheard of, for lower CR creatures. Some examples:
As can be seen, a sub-boss with AC 20 is available at CR 1. They are not common, but they certainly exist.
AC 20 isn't unreasonable and isn't that hard to hit.
At level 2-3 most characters should have at +5 to hit (+2 from proficiency, +3 from main ability bonus) on their 'bread-and-butter' attacks. That's a 25% chance to hit without throwing in some tactical play and use of special abilities (which are worth spending on a fight like this).
The list can go on, but these examples should suffice.
If the fight is challenging, it's time to burn special abilities. And then statistically a party of 4 should be able to average 1.5 to 2 hits per turn between them.
Aside, it's worth bearing in mind that sometimes an option is to avoid the fight altogether. Your party could try talk their way out of it, or even run in a pinch.
For a boss monster? AC: 20 is hard, but not impossible to beat at that level (assuming your best attack stat is +3, and you are proficient +2 = +5, which means you need a 15+ to hit it. This is a difficult roll, but should hit 1/4 of the time).
Also consider that something with that good of an AC probably doesn't have much in the way of magic resistance at that level, and HP usually aren't super high either. Between spells that don't interact with AC, advantage from ganging up on him (that high of a CR against the party and he's not going to have a ton of friends with him), and he will probably be challenging, but not impossible.
AC is not the only parameter to measure the difficulty of a fight. It depends also on damage per round, hit points, attack bonus and other abilties. If a creature has a high AC wrt his challenge rate, this will be compensated (for example with low damage per round). The DMG provides a useful table to check how the CR is calculated based on these parameters. You can verify this balance in the examples provided by Sawyer.
"Normal" is a pretty subjective kind of thing. It is normal in certain play styles, and among certain subculture groups in the hobby community. It is highly abnormal in others.
In 5e, it is absolutely possible to face a combat encounter balanced for a first tier character with an enemy of 20 AC.
If there is any degree of PVP combat, or if the DM throws an enemy built using PC rules at the party, it is possible to face an enemy with AC above 20 at first level.
You said the enemy was a town guard, which sets off some red flags for me. Many traditional DMs will give NPCs incredibly imbalanced stat blocks if they did not intend the players to fight them, as a method of manipulatively punishing disruptive play through PC death. A common tool in this technique includes outlandish lyrics powerful guards who can easily subdue the party and dole out an appropriate punishment.
