Yes to How To Talk - and if you want a good guide to positive approaches, there is a really great course on Coursera called The ABCs of Everyday Parenting.
It has a LOT more useful stuff than this but the gist of the ABCs course is:
Pick a few specific behaviours to work on
Work out what the positive behaviour expectation is (e.g. "tidy up after self/at certain time", rather than "not hitting when asked to tidy up") although with this one you can use not X.
Working this out then allows you to notice when they ARE doing it, which lets you praise them.
Also, specifically tell them what you want them to do (and it has tips about tone, wording etc which are more or less likely to be effective, which is useful)
If this isn't working there are more suggestions to target behaviours e.g. break them down into smaller steps, role play, check what you're modelling, increase rewards etc. There is also some guidance about how to use time out effectively though it points out that time out on its own won't change behaviour - you have to target the positive behaviour too.
The gist of the how to talk book (again lots more useful inside):
Look at the child's perspective/feeling behind their behaviour and help them express feelings in ways other than lashing out with behaviour, which should over time reduce lashing out tantrum type behaviour.
Try to meet them where they are at and see their point of view.
Ways to ask them to do stuff which feels less like pleading or being an angry drill sergeant. Mostly relating to playing and assuming competence and being positive about things.
Don't catastrophise e.g. if they are leaving pants on the floor at age 3 they're going to be a lazy layabout husband - it's probably not that bad and they will get there.
I also really like the Dan Siegel/Tina Payne Bryson books - I am not sure which of these are best for a 3yo - maybe No Drama Discipline or The Yes Brain? I like how they explain what is actually happening in terms of neuroscience and why they respond the way they respond to different things. I love that kind of thing and find it really helps me to have a why to do/why not to do something. But if you don't really care why he's doing it and you just want to know what to do to get him to stop, then the above 2 suggestions probably get to that point quicker!
Unrelated to these books - I do think some of the approaches which work well with toddlers start to sound annoying to 3-4 year olds. "Gentle hands" definitely falls into this category because a 3yo who is hitting is not trying to be gentle and expressing poor motor control - their aim is very much not to be gentle. Trying to soothe that away probably won't work - they need effective but acceptable ways to express frustration. It's also good to hand over some responsibility for choices to him and then let him find out what that means, rather than trying to manage every interaction that he has and then resorting to naughty step when that is failing.