As a Fiora player, it is true that all 4 of these champions have the upper hand against her.
Tryndamere isn't that strong early, but his W AD reduction brings Fiora down to his level. It is difficult for Fiora to win trades against Tryndamere as Tryndamere can utilize his 35% crit chance from his passive at all stages of the game while Fiora's vital damage relies heavily on her bonus AD.
Tryndamere is probably one of the only melee match ups that can beat Fiora constantly throughout the stages of the game.
Fiora can disengage from Tryndamere with her slows and her lunge, but that is pretty much all she can do against him. She will lose in a 1 v 1 fight if both champions have similar gold.
For Nasus, I was actually surprised at how well she beats Fiora when I first played against one. Fiora's vitals start off at 3% maximum health true damage. That is nothing compared to how well Nasus can sustain.
Champions that typically beat Nasus are those who can safely put out consistent damage without retaliation from Nasus such as Teemo or Quinn.
If Fiora tries to all-in Nasus, she might win the trade but Nasus will out sustain her with his passive.
Also, Nasus' ultimate counteracts Fiora's damage. A Nasus can ult after Fiora hits most of her ultimate vitals. The health he gains from his ultimate will not be penalized from the vitals Fiora hit earlier.
The same applies to other abilities that regenerates health or gives a strong shield. This is why champions such as Mundo, Nasus, Lulu, Shen, and Pantheon have such good win rates against Fiora since they can negate the damage from a few vitals.
Yi beats Fiora if he goes the infamous tank build. Yi deals physical and true damage as well, but in higher values. It essentially becomes a stat check - one that Yi usually wins. Yi just need to AA and can still output better numbers than if Fiora hit every vital perfectly.
If Yi builds glass cannon however, Fiora can usually get him by bursting him down with Grand Challenge.
Jax is my main, and the match up is surprisingly skill reliant. I get the feeling that you do not know Jax well; I can assure you that while a mediocre FIora can beat a mediocre Jax, a good Jax counters a good FIora.
Jax's E allows her to dodge Fiora's Qs and AAs, effectively rendering her useless while it is up. Moreover, Jax can decide when to close his E. The time frame that Jax can maintain Counter Strike is over 2 times the duration of Fiora's Riposte. Essentially, Jax can simply jump on her with E up and not close it until he sees that Fiora uses Riposte.
If Fiora decides to hold on to Riposte, Jax can literally just start walking away. Fiora can chase after him with Q, but she will end up tanking minion aggro.
No matter Fiora's decision, during the time Counter Strike is up Jax will take no damage from Fiora, while Fiora would have received at least 2 auto attacks.
In an all-in, there is no doubt that Jax would win. The reason Fiora appears to counter Jax is because she exploit Jax's greatest weakness, his inability to mitigate poke damage. Jax's counter strike has a 16 second CD at rank 1. This leaves Fiora ample time to sneak in some Lunges.
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do pretty darn well against her also..
is all she needs to 1v1 anyone
into the 
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