Here's my take on a triple planet system based on a clock motor. I intended this clock as a decor but is fully readable. 
Everything was printed using ivory white matte PLA and fits a standard clock motor as you can find on amazon. 
From the center to the outside, you get the seconds (smallest planet), the minutes (first planet) and the hours (same size planet with a ring).
The planets simply push on the little support. 
Happy printing !