Sconce lighting is IDEAL as long as it is at face level. You want the light to reflect evenly on your face if at all possible. So, lights on either side of the mirror, or, mounted directly over the mirror are ideal as long as they are at eye level. Bathroom bar lighting is unflattering and doesn't provide an accurate lighting for your face. Hope that makes sense. I'm not in anyway promoting RH sconces, this is just meant to give you an idea of the ideal type of sconce for the bathroom. You can also check out the National Kitchen and Bath association recommendations for lighting in a bathroom. A web search of "National Kitchen and Bath Association" will bring up their site. Here is a link that might be useful: Sconce

If you want to use a medicine cabinet I would probably stay with the 24 inch and use side lights. You'll need to keep in mind that a bar light above the medicine cabinet will need to extend enough to clear the depth of the cabinet, so finding such a light may be difficult. There are cabinets that have built-in lights so you may want to look at those. Check these out. If you choose a cabinet with built-in lights, I'd go with a wider than 24" cabinet. Other option: keep the light fixture you have and put in a framed, or unframed mirror that is a little wider than the lights. Here are some visuals.

Is the vanity in an alcove? If so, mirror the entire alcove and have the glass company cut the mirror for mounted sconces on the mirror. You could also do this if there's no adjacent walls or one wall, and you may have to. You have 38" to fit a mirror and two sconces (countertop overhangs an inch or so). I would want at least 3" between the sconce shade and mirror and the sconce and the wall, so 3" + 3" + width of sconce. I have a 24" mirror over a 36" vanity, which is a fine size for the mirror, but that leaves only 7" for a sconce between the mirror and wall. If you used a smaller than 24" mirror, the sconces might be too close together. I read that 30" - 36" is the optimal spacing for sconces. I always draw this stuff up on graph paper to check spacing and proportions.

I am not a design professional. I am a practical person who understands the need to be able to sit and do your hair or make-up. I also appreciate the need for drawers and see no reason to loose your drawers so you can have a knee space that is balanced in the middle when you have space for one at the side. It is stupid to loose function for design's sake when a better design could give you both. That is my opinion. I am wondering if there is a way to tweak your design to make it so the third mirror is not looking like a not-quite imitation of the other two. My thought is that the more different you make it, the better it will look. So make it shorter. You are sitting at this work station. You don't need the mirror to go as high as the others, maybe narrower. Your lighting will be lower and will be side sconces. You may even have the counter be not as deep. So you will be closer to the mirror. See, by making the work station different, it is a different thing on the side, not a third thing. If that does not work, turn it 90 degrees and put this work station on the side wall so that it is not in the same "view" as the other two.