Several versions of the large tube knot from the Escher woodcut Knots. I think this looks closer to the artwork than my previous thing, but unfortunately I couldn't get it to print cleanly. The top looks fine, but the bottom is a mess. I think someone might have better results on an SLA printer?

UPDATE: 12/04/2018 I tried reprinting the solid knot with one flat side against the print bed and the remainder held up by supports, and that works much better. I have updated the STLs with models that are rotated, but otherwise the same as before.

I've included both a solid-tube version and a hollow-tube version. You'll need to mirror the STL to get it to match the handedness of the original artwork.

Other trefoil knots depicted:
Escher trefoil knot (cruciform) by sombetzki
3 Knot by eduardoviruena

Modeled in OpenSCAD from images found at this informative page, particularly noting the images of Escher's paper ribbon models.