How was possible that Leo Bonhart, an "ordinary" human, was able to kill three Witchers? Was he just well trained, or had magic to help him as well?
Leo Bonhart was a well-trained bounty hunter and previously a witch slayer. From witcher.fandom.com:
Leo Bonhart (d. 1268) was a bounty hunter and previously a professional soldier from Ebbing. Known for his professionalism and expense to hire, he found pleasure in watching brutal scenes and inflicting pain on others. He was described as very tall but ghoulishly thin, with a prodigious grey moustache. He was said to have cold, expressionless eyes which were often likened to fish.
Thus Leo Bonhart is above "ordinary human" skill and expertise so it is very possible that he managed to kill 3 witchers during his life. Most witchers are not at Geralt's level and are weaker in comparison (as he was naturaly gifted) and the author doesn't reveal if Leo Bonhart had a fair fight against the 3 said witchers (meaning that he could've taken them by surprise, shot them with an arrow or they didn't use potions to enhance their power).
Well, we must consider two things about Leo Bonhart:
First thing, Leo Bonhart claims he killed three witchers. Nobody ever has proof: neither the characters, nor us, the readers. We can think a man like him, proud of his skills, wouldn't lie about such a thing... or would he?
Something Leo Bonhart also is: he's a ruthless pragmatic. And this bring us to our
Second thing: while he is clearly a very, very skilled swordsman, with a physique described as something out of continuous training and practice, the depictions of his fights also suggest something more: a lot of his victims are extremely afraid of him, some to the point of being terrorized by his mere sight.
As anyone fighting for sport or profession can attest, if you fear your opponent and think him invincible, you are already defeated. While killing half a dozen people may seem an incredible feat, it actually isn't if those people are so terrorized by you that they are afraid to attack or they will do so in a mindless frenzy: And this is actually how most of the times Bonhart defeats his enemies: by striking fear in their hearts. Some he kills thanks to his skill, like in the last part of Lady of the Lake, but mostly he defeats his enemies by fear alone, the sword just doing the dirty work. So, his "killed three witchers" boast can just be part of his "legend", that he built to make others think he's invincible. So that he is.