Just buy one customized sized mirror at 170 inches wide by 60 inches high and hang the midline at eye level. https://www.dullesglassandmirror.com/customize Rectangle You can look into ways to frame it depending on your design style preference

This all depends on what decor style preference you are going for.... traditional, modern, farmhouse etc? If you don't know your style, take the free quiz here...https://www.dsasociety.org/resources/interior-design-style-quiz/

So do I do three mirrors all the same size? Or do I do two the same size over the sinks, and a different one over the makeup area!?

Not sure, I need a better, simpler drawing with labels to see placement

You could do three separate mirrors the same size. Or you can use a different shaped mirror over the makeup area. Here are some photos of each style:

It's really personal taste. I prefer using the largest mirror that will fit in the spaces. The individual mirrors are lovely too, and easier to change if you get tired of them.

Since that looks to be a seated area, I would do a nice wall mounted light up round mirror and then have 2 mirrors over the sinks