I need a D3 often enough that it makes sense to have a dedicated one, and I didn't like the available commercial options and/or didn't need 5 of them. There are some very clever designs out there, but in practice I found most of them didn't roll crisply, so I ended up just printing some duplicate-face dice.

These are trivial to make yourself or to customize from existing models, but even easier to just download and print, so I'm throwing them up here.

Customized version of https://www.thingiverse.com/thing:868110
Customized version of https://www.thingiverse.com/thing:5189255

They're about the right size for standard dice, but scale as needed for your use case.

Print at 100% infill for the fairest dice and best roll, remembering that 3D-printed dice are never guaranteed to be perfectly fair.

Customizer options

For the 6-sided:
scale=.85, depth = .75, cube_size = 20, curvature = 1.4, font = Aladin

For the 12-sided:
size = 20
letter_ratio = .45
depth_ratio = .03
sides = [1,2,3,1,2,3,1,2,3,1,2,3]
rect3 = [[0,+g,+1/g],[0,+g,-1/g],[0,-g,+1/g],[0,-g,-1/g]]
rect4 = [[+1/g,0,+g],[-1/g,0,+g],[+1/g,0,-g],[-1/g,0,-g]]
rect5 = [[+g,+1/g,0],[+g,-1/g,0],[-g,+1/g,0],[-g,-1/g,0]]