My first thought was Forge of Fury as well. I even made a couple DMsGuild modules that lists all the statblocks and such to make running it easier. I think it was called Forge of Phandalin to combine the locations but skip all the opening parts getting to 3rd level and you just have the dungeon itself. Likely need to chop a few encounters off for time though.
Another idea is the MCDM Delian Tomb starter. I found free online the adventure and printable battle maps for using minis. I ran it for scouts ages 9-12 and it was fine. I expanded a few areas and skipped most of the town setup and it lasted 2.5 hours.
Forge of Fury takes easily four 4-hour sessions to complete. There are four large dungeon levels, an optional 5th one, and then a dragon lair.
I agree with those suggesting Dragon of Icespire Peak as a great source of combat-heavy, minimal-story one-shots, especially for levels 3 and 4. Axeholm as written is more for level 5-6 though. The quests with the ankhegs or the ranch attacked by orcs are better for level 3.
I dont know much about Adventurers League at all. Though, Pathfinder Society has 100's of scenarios meant to be played in a 3-4 hour window. Many are free or as little as a few bucks. Do they have similar adventures you could look at for AL?
The PCs want to kill a crime lord for killing a PCs' child or whatever. They find that said crimelord is hanging out in a tea house (or something) that night. The PCs will then go attack said crimelord. Give the crimelord a higher level bodyguard who's a very bloodthirsty teenage girl with a ball and chain. Also eighty eight or so grunts (a generic level one warrior type will do for them all) with swords who rush in after the body guard is killed. Make the crimelord a duel weilder so that she will fight the PCs too. Bonus points if there's no honor or humility during the adventure.
The Crimson Oubliette. This is a unique dungeon that really is nothing but combat. You'll find it in the 3rd(?) issue of Kobold Press Warlock magazine.
This is a giant pit full of acidic slime intent on eating you. However, the slime is averse to fire, so it will pull back from a torch twenty feet allowing you to explore the pit without letting you see the whole thing at once.