IME 3.5 is a turning point where other children start to settle down, understand the rules you've been trying to teach them for the last 2 years and generally act a bit more human whereas the ADHD kids just ramp up like they have done every 6 months or so from the age of 2. It's exhausting because at this point you're wondering when the phase will end and if you're not careful you can take it for granted that it keeps ramping up, rather than noticing that other children have come out of the phase and yours is going in the opposite direction.
DS1 was just mad, wouldn't sleep until some ridiculous time of night, used to take his clothes off as I was putting other clothes on, argued everything (and was smart enough for it to make sense!), had to try to control everyone, wouldn't let me talk to other adults in his presence, extremely anxious, incredibly big feelings in every direction really. He would hit me, throw things. He was like the kids in the "before" stage of Supernanny. In hindsight, I was extremely young when I had him and struggling with my own undiagnosed ADHD so I didn't help matters at all by having barely any routine or structure. It really damaged our relationship because I got completely burnt out and it took a long time for it to recover. He did calm down a lot once he started school aged nearly 7 (we live abroad). I think his ADHD is very mild. As a teenager it presents almost exactly like mine. If I hadn't been diagnosed myself, I don't think it would have been picked up and I think if he'd had a more sorted parent as a toddler, he wouldn't have gone through the tricky stage.
DS2 seemed like a totally different beast and I thought aha structure does help - which it did, but he still struggled even so. I feel like even now, although he is now on medication and this is less the case - when he's not on medication, it's as though he does not so much "have" emotions, but rather that emotions are something which descend and take total control over his entire being, and then leave just as suddenly, and he will be perfectly happy and cheerful and wanting to relate to me normally while I am utterly reeling with aftershocks from the rage monster he was ten minutes previously. DS2 had more issues at Kindergarten and school. He was also 7 when he went to school and it was a complete disaster immediately. TBH he has a lot of signs of AuDHD. Everything is much more sensory and he is very much more about things needing to be predictable and follow rules. DS1 has never cared if there were rules and would quite happily live with no routine or structure without a care in the world. DS2 would utterly fall apart.
There is a particular high pitched laugh that both of them used to do when they got into the dysregulated-silly state that by the time DS2 started doing it would instantly trigger me into a fight or flight response because of what I had learnt to associate it with in DS1. However because I knew what was going on with DS2 I was much less burnt out and I have maintained the connection better.
Best resources for this age IME
Yale Everyday Parenting course (on youtube)
When Your Kids Push Your Buttons by Bonnie Harris
Conscious Discipline (the original stuff or Mr. Chazz)
Google "Sensory Seeking" and drink all the info you can find
Structure and predictability and responsibility - get them to earn stuff and find small things to praise for. Give them REAL tasks that matter for the family or for themself. Do not do everything for them, allow them the chance to feel capable and have autonomy in this way.
Break EVERYTHING down. If you're not having any luck with behaviour, break it down into much smaller stepping stones.
Rule of thumb for ADHD: Praise for 25% correct. (With a 3yo you may wish to praise for 10% correct). If you're waiting for 80% correct they give up before you get there. ADHD brains are "leaky" with the transportation of reward chemicals like dopamine, so the sense of achievement most people naturally get from seeing progress doesn't happen the same. You can drip drip rewards/praise to mimic this effect. Don't stress about destroying intrinsic motivation, they barely have any anyway unless the task is inherently motivating, and while following your passions is all well and good it is fairly rare to have a passion which doesn't ever involve any boring grunt tasks. Adults with ADHD have to figure out how to externalise this process, so you may as well start now.
Keep any consequences extremely short lived and minor, contrary to current fashion it doesn't matter if they are related or "logical" as long as they are predictable, consistent (meaning the behaviour you do not accept is consistent) and not scary. If you're struggling to find a related consequence for everything then stop, you don't have energy to spare and anyway, it tends to result in the severity of the consequence being linked to your own emotional state, which some children find confusing and even scary. If you're finding you feel you want to increase consequences, your expectation is probably too high, try breaking down the step you're asking them to make instead. (The Yale course is excellent with how to do this).
Emotion coaching/talking about emotions is fantastic, do this as much as you can in an authentic way (ie not in a way that makes them sick of it) the How To Talk books are brilliant for including emotion language in a real way, but you can also do it in role play, when reading, when watching TV together.
Be EARLY not LATE. This is language from Ross Greene who is fantastic, but essentially the concept is to be proactive in managing behaviour and any decent ADHD expert or even parenting expert will say the same (although I find it more essential in ADHD parenting because trying to teach them anything through reacting in the moment does not stick at all, IME). The conflicts you have with your child are usually highly predictable, ie you can probably name 5-10 areas which cause difficulty on a weekly or more frequent basis. So sit down and choose one and write a plan. When does it happen? Where? Who does it happen with? What can you do to set them up for success? What's the expectation you have, and is it realistic? How are you going to work towards it? Most ADHD experts and the Yale course advocate for working directly on the skills and expected behaviour by practising in a more controlled environment and gradually increasing the freedom as they gain competence with it. Ross Greene encourages more of a problem solving approach which assumes that since you can't magically make your child have the skill that they currently don't have, there are ways to meet them where they are at which can be used on a temporary or permanent basis. His books are great to read but usually frustrating when your child is 3, however his concept is brilliant and I do think long term it is essential reading for any ND family.