I couldn’t speak for History, but I’ve glossed over Economics and it looks very easy, so it could be a good option- there is a book on Amazon by Duncan McMaster.
I’m also sitting Mechanics this year, and as I’m sure you’ll know, it goes very well with Maths and Physics, about 40% of the marks being accessible solely doing those, I’d say. It starts to get difficult once you’re looking for those remaining marks- heavily proof based. The closest thing from the Higher Maths course I could compare it to is the optimisation part 1/proof questions; the questions in Mechanics require a similar amount of problem solving, though are slightly harder. If you can’t do those, I think you might struggle with mechanics. It is possible to self teach, as I’m doing, but I’d imagine it’d be easier if you were in a class.