I would either do one beveled mirror that spans both sinks or have 2 mirrors.

3 mirrors does sound weird, either one or 2 would be good

I prefer to have sconces on either side of mirrors in bathrooms. That gives better quality light for grooming. If you do that there is less space in between to worry about. You can always put up glass shelves if you feel the space needs something, but then you have to be artistic with what you display and it will need to be cleaned regularly. Wait until everything is done and think about it for a month or two before you do that to see if the space grows on you the way it is.

I had a bathroom with a double vanity similar to what you describe - lots of room between the two sinks, the vanity ran the whole length of the room (shower/bathtub and toilet on other wall.) The vanity had three mirrors above it; each was a medicine cabinet. One mirror was over each sink and one inbetween. These were probably installed in the late 80s. We appreciated the three medicine cabinets because there was not a lot of closet space in the house and most of the room under the sinks was taken up with towels, cleaning products, etc. That said, I was not a huge fan of the three-mirror/medicine cabinet look (with lights above each). If you are not pressed for storage I'd go for one big mirror, or two mirrors. Three will make your wall feel chopped up. Either one mirror or two will make it feel more spacious and more symmetric, imo.

We did 2. For the mirrors, we used the same wood as cabs with beveled mirrors,