You will get conflicting answers because there are multiple schools of thought on what causes unwanted behaviour and therefore how to reduce it.
Three is bloody awful anyway whatever approach you go for. And all kids struggle when they have a new sibling. 
The approach/school of thought in 123 Magic is behaviourism and Antecedant/Behaviour/Consequence. ie, the idea that your own behaviour as a parent is what influences your child's behaviour, and you can change their behaviour by making your requests clearer, improving your communication and providing immediate feedback about behaviour whether you like or dislike it.
I don't actually like 123 Magic, but it is evidence based, easy to follow and it does work for most children. If you like the sound of this, there is a bit more of a modern take on it on Coursera called The ABCs of Everyday Parenting, IMO it's much better - far more focused on the positive aspects and encouraging a replacement behaviour that you prefer, but this is basically the same theory just presented in a slightly different way. I find this is less heavy on the bits of 123 Magic that I dislike. (But also me disliking it is completely incidental, I'm no expert, just a mum who reads a lot.) It's a free course and most of it is 10 minute long videos so easy to follow even if you don't have a lot of time. It is also MUCH less focused on time out being something punitive away from you, and you can also use every technique except for time out, if you want to. So you could give it a go.
The other main approach/school of thought is that behaviour is communication, ie, when things are going well, generally children don't behave badly and are open to more collaborative or cooperative approaches or listening to explanations and reason, and will pick up on social norms and adult expectations fairly easily and go along with them. (Which is exactly why, 90% of the time, it doesn't matter if you do behaviourism "wrong"). When they are not doing this, it's a sign that something else is going on for them.
Unfortunately it then becomes really spread apart - this school of thought does not have a nice, neat answer like behaviourism tends to.
So you will get one person who says OK, when they are behaving badly it's because they are seeking connection from you. Spend more time with them and the behaviour will stop.
Or they are having "big feelings" and they do not yet know how to express them in a more appropriate way. Validate their feelings and they will gain words for it and the behaviour will stop.
Or they are stressed out by something e.g. a new sibling, a new school. Wait for this to pass, provide extra reassurance and the behaviour will stop.
Or it is a skill gap e.g. speech development, social skills. Give them more support in that area and they won't struggle so much and will catch up and the behaviour will stop.
Or it's sensory. They are overloaded and you need to reduce their sensory input and they will be less stressed out and the behaviour will stop.
(Add dozens more potential reasons.) The problem with all of this is that actually, it's quite likely that it is true that a child who has a lot of challenging behaviour is struggling with something. The notion that "behaviour is communication" is probably true. The problem is that it is usually very difficult to work out exactly what it is which is causing a child's behaviour, and plus it probably is not one neat perfect answer - it is most likely a combination of factors. Children do not come with handy interpretation manuals - which is exactly why behaviourism goes directly for the behaviour, and not the root cause. That makes it very measurable and very easy to apply because it is always the same, though it is also the major criticism of behaviourist approaches. Some behaviourists argue that it does not actually matter what the root cause is. Personally, I think this is short sighted. However, I can recognise that it is common for authors/experts/practitioners (and particularly, influencers) in the "behaviour is communication" sphere to laser focus on one particular underlying cause and insist that all "bad" behaviour is caused by this. In general, any stressor you work on with a child - any connection, any skill gaps, any emotion coaching - will have an overall stress reduction effect and it will likely help, but it is probably not the whole picture and you can focus overly on feelings OR connection OR reassurance or whatever it is but if it is actually that your child is struggling with sensory overload and confusion because they find other people extremely unpredictable, then this probably isn't going to be enough and you will run into problems. And then if you're very unlucky you'll run into a lot of guilt/shaming from people who are desperate to convince you that you're not connecting, reassuring or accepting enough.
Although in general I prefer the behaviour is communication approach and tend to think this is more complete and better - it is also slower, it is more complicated, it can have counter-productive results particularly short term or if you are over-interpreting messages from social media. I can see that it actually is useful to take at least some of the concepts from behaviourism and try them out. This will give you a clearer picture at least whether your child is responding to unclear messages, or whether they are actually struggling with things (because some children have so many things interacting that straight up behaviourism will escalate things rather than calm them.)
If you think that your child is struggling to gain skills that other children their age have, then you can speak to your health visitor or GP. Speech/communication is a common one that can need support at this age.
If you like the How To Talk tips but struggle to remember them in the moment, there is a useful app (it's the Mythic Owl one).