Purely from a vram perspective. A triangle is just 3 vertices and some edges. A few bytes of data.

A similar 2d image of a triangle would likely be thousands of bits of data.

The 2d probably uses a little less per pixel. Whereas the triangle has slightly more expensive compute per pixel cost. But modern gpu have dedicated hardware for that.

3d is probably more expensive compute but cheaper vram usage. At least to a point and a certain set of conditions. Barring things like textures and others.