It depends on the animation and which animation method you are using. In pose to pose method.(Similar to what you describe) You would make your key poses( A and B in your case). Then do a break down pose(your C) by going to frame 50 and moving the cube to where you want it and inserting a keyframe. You will probably want to change the handle type to vector( with mouse cursor in the timeline/graph/dopesheet editor press V->Vector) so the cube doesn’t slow down and speed up as it reaches/leaves frame 50.
Side Note: once you are on frame 50. A quick way to move to C point is to use ALT+G to clear the location and then G->X and move to C point. Then press I to set the keyframe.
