After Effect's timeline curve editor, in 3D, SUCKS! It keeps getting worse every year. It can't even perform simple position tasks. The curve always projects pass the intended keyframe and there is no way of customizing it.
Any one else have extreme frustrations with the pointless curve editor that is in After Effects. After Effects crew, you need to take a lesson from the Maya crew. Their curve editor is amazing. I can't get anything done with YOUR curve editor. Especially when I am working in 3D.
Right-click on the position values and choose to Separate Dimensions.
That should help somewhat.
You also might try (if you haven't already) switching the graph type from speed graph to value graph.