How to perform an action on the start of a 3 finger tap on a trackpad?

I want to perform an action on the start of a 3 finger tap on a trackpad. I found 3 finger tap but that only works at the end. I want "trigger on key down" but for trackpad. How can I do that?

That's not possible with BTT, sorry!

Do you know of any way to do it (without BTT)? I got what I wanted working in an NSView with some Swift code, but I don't know how to intercept global trackpad events like BTT does.