Yes remove the mirrors. You can place a large piece of art - eg. a colourful canvas in the large niche and sets of stacked framed art in the smaller ones. (either b&w photography or perhaps botanical prints)

You could frame out & install drywall in the largest one. (Don't know how deep it is). & remove the mirrors in the side ones. Can you give us photos of each niche closer, please?

I think I'd leave the two mirrors on the sides but I might remove the biggest one.

Your hanging light is fighting with the round object in the back wall. I do like the idea of adding a grouping of photos or prints on just the back one & removing the side ones by drywalling over them.

Niches can be such a difficult design element.

These niches were popular in houses built in the late 80s-early90s when tromp l'oeil murals were popular, and that can still be a good option if you want to keep them. I have one and framed pictures don't work well. Two dimensional pieces do better. I think I'd seriously consider drywalling them in though.

If you drywall the niches in, you'll be dismayed how plain your room becomes. Not sure why someone would say your fixture is fighting with the arch...........it's not. If you 86 the mirrors, you'll want to at least paint the niches in some type of contrasting color. Are there other arches in the home?

Yes, there are two narrower niches that are also mirrored flanking the entry to the kitchen.