Although I agree with this, in practice I'd expect to use scissors a lot more often. Which I'd prefer in a tool depends on whether I am focussing on every day use, or on emergency use.

It does partly depend on what else I've got. I carry a tiny Vic Rambler which has tiny scissors, and also an LM Juice which has more meaty scissors. I find the Rambler scissors much better for working with my fingernails than the larger Juice ones, and as I have longish nails that's crucial for me. Also, they are about as good for cutting plastic or paper. For a lot of domestic jobs, just about anything sharp will do in my experience.

The Juice scissors are much better at bigger jobs, such as cutting someone's shoe off or clothing off, as might be needed for first aid or rescue. I'd much rather use scissors for that than a knife (especially if the knife wasn't designed for the job and was pointy). However, as you say, a knife would do in a pinch and I probably wouldn't cut up my patient too badly by accident, so for that role the file would cover more bases.

I live and work in an urban environment, so a file that can cut metal is more important to me than a wood saw. It's especially valuable long-term if it can be used to sharpen another blade. So I guess my ideal 3-tool SAK would be blade, scissors, file; no saw.