Update keyframe animations only every third frame in After Effects
I don't know if there is an easy way to do it directly.  You could do some modular math in a script for the keyframe positions, but that's kind of complicated and non-user friendly.  An alternate approach would be to do an additional nested composition.  

Make an animation composition for your character that is at 8 FPS, perhaps bring in the main composition as a backplate while putting it together, then remove the backplate and import the animation composition at 24fps as a stationary composition.