Here is what stands out as awkward.The arrangement covering the bottom corner of the mirror. What do you want the mirror to reflect? Either a large full bouquet or a long low one not obstructing the mirror. The chain and books add nothing to reflect any style aesthetic, just trying to mimic similar displays from a store or magazine, mostly because the balance is off. Do you use the buffet as a buffet that one might derive from? If so, maybe you have a pretty soup tureen or bowl on the buffet instead. There are many beautiful photos on houzz with examples of buffet styling.

I don't agree about the bouquet; I like that it's reflected in the mirror. It makes it appear fuller and 3 dimensional. I love the color. Perhaps look for a more interesting vase and something instead of books and chain. The chain does look contrived...there would be no reason to display that. Not knowing what you own or like, if I were you, I would look at pictures of chests and buffets that are styled in a fashion that pleases your eye and do something similar. It really doesn't look bad, but could look better!

I was also told to do things in 3s

I like what you have placed on the buffet, but overall there is something stark about it. It may be the darkness of the buffet and the matchy-ness of the mirror, adding to the overall heaviness?

IMO a nice piece of art there no mirror and IMO the sconces do not look good too much in a small space , the reflection in the mirror adds to the eye clutter

Lots of good advice - I think the glass vase shows the stems as clearly faux, I‘d get something ceramic and interesting and maybe a skooch shorter (cut down the stems accordingly)

I’d also want some more texture, what about grouping the other items in a low rectangular basket? then maybe add a sweet family photo in a frame on top of the books?

Love the sconces. Agree that the chain must go.

In general, I do not prefer mirrors in dining rooms. Without seeing the whole room, I don't know if this arrangement presents this issue, but I don't think anyone wants to see themselves eating. The mirror and the buffet are also too matchy-matchy as Amy K said. Art would be an opportunity for a more interesting arrangement. There's also something about books in a dining room that doesn't sit right with me. I think a pottery or china object, or a basket would work better.

Using 3’s won’t result in a balanced effect, unless proportions and heights are considered. Maybe try your mirror vertically, as it feels squeezed in. Look on line for an overall style/feel, buy items (that can be returned), and keep testing until there is a sense of harmony. Try HomeSense and Amazon.

Vs:

Agree with Maureen and Celery…hang mirror vertically.

Agree. Vertical really helps; it competes less with the buffet.

Just an off-tangent comment: I was taught the 3 Rule for art projects and for arranging flowers and plants. Actually the Rule that I learned was to use Odd Numbers, not just 3s.

The sconces are too demanding. They draw the eye away from the buffet and it’s decor, especially as they hang well outside the parameters of the buffet. Hang the mirror vertically, and perhaps use sconces that are smaller, something like Maureen shows in her example .