How can I use these triangles for home decor?

They look like a pyramid shape. Why not make some concrete shape and decorate them.

Those are cool! You should punch holes in the corners and string twine through them. Paint the cardboard and add succulents for a cool modern plant hanging.

Maybe something like this...

Use them to hold a project off the table when painting it.

I think you could make adorable and salable lamp shades from each triangle. Paint them in colors and designs you like. Use an ice pick to puncture holes in either lines, shapes or random spaces. Stop there, or install a whole light kit.

Omg ! I would paint them and string together long ways. Then I would add cheap dollar store chimes to them. I would hang inside where the darn cat 🙄 or my little one could reach. It keeps both entertained.

I saw similar triangles decorated with flowers on the wall in one restaurant. You can take either artificial flowers or live ones (although if you choose live flowers, you will have to take care of them). This element of decor is suitable for any apartment design, but the main thing is to choose the flowers that will be combined with it. Recently, My sister renovated her apartment and saw a similar idea as you have on https://www.goodhous.com. She just insanely loves flowers and constantly tries to combine them with the style of her kitchen.

Hello,

Pyramids are supposed to improve anything they sit over. Place over Glass of wine for 1 hr. and see if it's better than one not covered with Pyramid! Once decorated to taste. Use to hide things, use as Lampshades etc.

You could use them to create a geometric wall art display by arranging them in a pattern or shape on a wall. Another idea is to use them as unique coasters or trivets for your dining or coffee tables.

You can add a bit of originality with some animal themed decorations from https://untamedcreatures.com/.