Prefab hierarchy...

SomeGameObject     // <-- this is 3D map marker game object
   ChildGameObject   // <--- change pos/rot/scale of this game object

You have two ways:
1. OnlineMapsMarker3D.altitude
2. Wrap your 3d marker to a new GameObject, create a prefab from an external GameObject, and use new prefab for 3d markers.
In this case, you can modify the position, rotation and scale of the internal GameObject.

'OnlineMapsMarker3D marker3D = control.AddMarker3D(lon,lat, prefab);'

This works great, but I want to adjust the Y position of the 3D marker(game object) in the scene.  For example, the marker instance transform position based on the lat/long might be (10, 0, 15).  I want to change the Y but can't figure out how.  Here's what I've tried:

// Get the 3D marker game object

GameObject currMarkerGO = currMarker.instance.transform.gameObject;

// Get the position vector

Vector3 pos = currMarkerGO.transform.position;

// Change Y pos

pos.y = 3.5f;

// Update marker position in scene

currMarker.instance.transform.position = pos;

Thanks.