The combat matchups were more about plot than training.
Karli: We don't know anything about her past, she may have a black-belt in something or other as a teenager. Or not. She uses her size to her advantage, ducking, dodging, etc. In the first encounter with Bucky he was off guard thinking she was a hostage. Most of that fight was Buck and Sam against multiple Flag Smashers so it's hard to say. Later fights are driven by the power-of-plot.
Walker: Military training does not focus on hand-to-hand and acrobatics, much less shield slinging. Yes you get some, but 99% is firearms focusing on team co-ordination. Walker would obviously have received additional training for the CA job, coming from trainers-who-were-not-Steve-Rodgers for a couple of months, easily allowing him to out-punch anyone other than hot Wakandan women (sarcasm). Considering Bucky had WWII commando training, Hydra assassin training, dozens of missions as the Winter Soldier, gone toe-to-toe against Steve multiple times, he and Steve toe-to-toe against Iron Man, and then there's Thanos' hoards, I'd say Walker lasting 20 seconds against Bucky and Sam would be optimistic. The fights were driven by plot, not realistic training/experience levels.
Bucky: Seriously under-powered compared to his performance in CA-TWS, but this is plot-driven, and possibly excused by de-programming in Wakanda which may have repressed much of his training and experience.
Sam: Frankly I've always wondered how someone trained and experienced in "para-rescue" has a level of combat prowess that enables him to beat up pretty much anyone, but I love the guy and want to watch his shows/movies. The amount of practice he put in during episode 5 helps but it's still a stretch. I'd believe, say, Clint Barton could have stepped into the role that quickly. That said, I enjoyed the show and want more.