You should consider using side sconces for better lighting of your face. You have room for a sconce on each end and a double sconce between the mirrors. I think you'll have a more balanced look too--form and function.

You have a wide space between mirrors to fill. That's why the double sconces are nice. With over-mirror lighting, I would use as large of a mirror as possible to close the gap between mirrors. With 31" sink bases and ~5" of space between the vanity and wall, you can easily use a 28" mirror. With a 28" mirror and 3" of space from side of mirror to edge of vanity, there's still 22" of space between the mirrors. How will you fill that? Maybe a tall plant like an orchid? I don't care for a towel ring or art between mirrors.

You could also do triple mirrors. You could do all 24" mirrors or a smaller center mirror. Often these are framed to look like one unit. You might do a three-light fixture above the sink mirrors if using a smaller center mirror. If using three 24" mirrors, you might use a two-light fixture above each mirror. I would draw your options out on graph paper before proceeding. That's what I did to address your problem.