Issue
In Motion 3, text on a path is possible even when that path is a 3D curve. When text follows a 3D path in this fashion, the glyphs tend to automatically "point up" so that they stay oriented with the global up direction. This behavior simplifies working with text on 3D curves.
In some cases you will want the glyphs to stay upside-down as they follow a path, rather than flipping over to point upwards.
The following illustration shows a case where automatic glyph-flipping is not desired.
We can see that the artist intended the text to follow the curve upward through the arc, but instead of the letters remaining upside-down, they flip over and look backwards. As the letters pass where the path aligns with the vertical axis, they flip to point upward, rather than just flowing with the path in a simple fashion.
Solution
This behavior can be controlled by adding a couple of points to the curve and by flipping the normal for curve points where text is upside-down.
In the illustration below we can see the same project as above has been improved by adding control points to the curve at the inflection points (Note the new points on the curve where the path goes vertical).
After adding new points to the curve, control-click on the first point where the text is flipping, and choose Flip Normal from the shortcut menu. Follow along the curve, and Flip Normals for other curve points where you want the text upside-down.