You can arrange the three colors in a triangle, which defines a plane, is treated like a circle in color theory, and looks like a weird stretched out "U" when put in a perceptually uniform space (stretched towards green, which we see best, and flat on the magenta side). Perceptual color spaces combine this with luminance, which is sensed by rods instead of cones.

Tetrachromatic colors could be placed in a tetrahedron, defining a space and being approximated as a sphere. A perceptually uniform representation would probably be curved on the three edges that agree with the spectrum and flat on the three that don't. Green is still probably the biggest. Then you'd add luminance as a fourth dimension because it's still rods.