I don't think your parenting is bad. It sounds to me like your LO is easily overstimulated, and this is what is causing the strange behaviour that makes him seem bad/angry, and also why good, ordinary, sensible parenting is not working.
That's not caused by parenting, it's just temperament/the way someone is born. But he is NOT bad. I promise you. I listen to a lot of experts who work with behaviourally challenging children and ALL of them, literally every single one of them, they all say they have never met a bad child. A challenging child, yes. Parents and teachers struggling with this child, because they don't have the right tools, absolutely. Children acting out because of something that they can't control, yes. But once you get the right pieces of the puzzle in place, it gets better.
ADHD does not "cause" violence, but can cause for example:
Emotional dysregulation - they feel feelings much stronger than other people
Difficulty with impulse control - less able to pause and think about consequences before acting
Less awareness of social cues - so other children's behaviour might not make sense to him.
Now bear in mind the fact that even a totally normal 3yo can quite regularly become overwhelmed by their emotions, struggle with impulse control, and have not learned much in the way of acceptable social interaction.
If he feels frustrated, or threatened, or just annoyed - the mature response is to say "Please stop doing that", "Leave me alone" or walk away, or tell an adult. But it is a very rare three year old who is mature enough to do these things every single time. They tend to resort to immature coping strategies fairly regularly, because they are immature - they haven't built these skills yet. Thumping or kicking someone or throwing things or screaming or blowing a raspberry at someone are all examples of immature responses.
So yes, in a very roundabout way, ADHD can show up like violent impulses because someone with ADHD will be more likely to be frustrated/irritated/threatened by others, they are less likely to be able to moderate their response, and they are less likely to know what the correct response is. But if you support a child with ADHD by helping them learn how to replace those impulses with behaviours you like (like saying "leave me alone" or asking a teacher for help) AND you help them by ensuring their environment is not so overstimulating or they are getting the right amount of sensory input for instance, or you ensure they are more closely supervised so that an adult can help moderate child-to-chi then the violence will probably stop or hugely reduce. It isn't like it's a part of ADHD, but it can be a sign that the ADHD (or other issues) aren't being managed very well.