if you are using/can upgrade to 4.4.x,
look into Mesh.worldBounds, particularly wordBounds.overlap(someOtherWorldBounds).
Example (away3d setup elided):
// setup objects and materials
cubeMaterial:ColorMaterial;
cube:Mesh;
sphereMaterial:ColorMaterial;
sphere:Mesh;
collideMaterial:ColorMaterial;
cubeMaterial = new ColorMaterial(0x3333FF);
cube = new Mesh(new CubeGeometry(), cubeMaterial);
cube.x = -100;
cube.showBounds = true;
sphereMaterial = new ColorMaterial(0xFF3333);
sphere = new Mesh(new SphereGeometry(), sphereMaterial);
sphere.x = 100;
sphere.showBounds = true;
collideMaterial = new ColorMaterial(0x33FF33);
in your enterFrame handler:
// process your object movement here
if (cube.worldBounds.overlaps(sphere.worldBounds) cube.material = collideMaterial;
else cube.material = cubeMaterial;
view.render();