I've been doing some colorwork recently, and needed to be able to hold all three strings on one finger without them sliding together, so I could easily pick which one to knit for each stitch. Intended to be used Continental style. Symmetrical, so it can be used right or left handed.

Intended future updates: wider range of sizes (if you need a particular size, let me know), and 4 and 5 stringed versions.

Size is the inner diameter of the ring. The two styles differ only in whether the string channels protrude from the face of the ring, or are rounded off to keep the ring a circle. I find the version with extended channels a little easier to string, but your experience may vary. Photos are of the 20mm versions, which are slightly large on me.

Print without supports. The steepest overhang is 40 degrees (from horizontal), and there are no bridges. You may want a brim as the contact area is fairly thin. (My first prototype spaghettied on the third string and I've printed the rest with a brim. It removes easily.)

You could print it in multiple colors to match your strings by swapping filaments at layer lines.