Well, I think you are maybe being a tiny bit overcautious: 3 year olds don't generally stick small things into their mouth to explore them like babies do.
Having said that, I am sure I paid more attention to that kind of thing with DS1 than with DS2 and less again with DS3 and DS4 (17 months) might be living on a steady diet of tiny Lego pieces, for all I know, as frigging Lego seems to be breeding and trying to take over our house!
I think roller blind stings are a totally different kettle of fish and must be cut/kept short/out of reach of DCs.
Plastic bags - my lot have never shown much interest in them; don't have them in their rooms, because they have no use for them.
Magnets: in 20 years of doing medicine of which 10 years where spent in A+E I have seen many, many magnets in all sorts of kids' orifices (mainly noses and ears, you'll be glad to hear) and of course they swallow them, but have never seen or heard about the sticking-together-problem.
The only fatal choking incident I was ever involved with
was a 6 year old who choked on a Werther's Original sweet: they are the perfect shape and size to obstruct a small airway. They have been banned in our house since, and I am usually a stong believer in benign neglect and allowing children to learn by making mistakes. Not that one!
Small button batteries: if swallowed need to be retrieved as they can corrode and leak btw.
OP, small children have a very good gag/cough reflex and it is amazing what they do get away with, not how much happens to them
.
You could keep all the marbles in the world away from her and she could still eat/choke on a pebble. IMO teching about dangers is much more productive (I always thought the King burning all the spinning wheels in Sleeping Beauty - well, the Disney version - was sadly deluded: he'd've better taught his PFB where the pointy bit on the spindly was, so she could avoid in
).
Anyway, I agree with whoever said, we all make parenting decisions and judgement and do the best we can for our kids. Anxiety can be a bugger to live with.