my style has been to explain that there's really no such thing as ghosts or monsters, they're just stories and toys
I think you're already doing it right.
My wife's preference is to not even bother with theory, and just frame things in practical ways he understands: "The ghost lives in the shop with his ghost friends. It doesn't like sunlight and it's not going to come to our home."
The two of you need to get your stories straight - you need to agree an approach with each other. Just for you to flatly contradict one another is going to confuse him and potentially make things more scary.
It might help for you to talk it over with him in more detail. You might have to repeat the same message in different forms over a bunch of different occasions:
"Some people like to tell scary stories, but that doesn't mean they're real."
"If you don't like that kind of story, I won't tell you any of them. That's ok."
"That's just pretend. Those people are dressing up to play a game."
"We're not afraid of pretend XYZ's, are we? Let's practise telling them to go away: GO AWAY, SILLY POO-POO XYZ's!"
As in that last example, you can combine your approach (they're just fiction) with your wife's approach (within-story reasons why the scary things aren't a threat) by making up stories together with him - and emphasising that you're making up a story together and it's all just pretend and so on - and in the stories the ghosts are totally un-threatening: you and him together call them rude names (toilet humour is good here, because humour will make him less scared, and nothing is more humorous to a 3-year-old than bodily functions) and they run away scared of you. And then emphasising at the end that the whole thing is just a silly story and they don't exist and aren't real. That kind of approach can help him find the courage to not-be-scared even of things which he knows aren't real.
We've found these useful:
- "Thomas & Friends" episode on Halloween (book and TV episode both available),
- Angelina's Halloween (be warned - extremely charming illustrations).