i've written a method to merge Animated3D's in different groups. but to work properly it needs groups have identical skeletons. looking at your files, i see skeletons differ in number of joints.
are you sure, your artist have one skeleton and attach all objects to it? it must be so, otherwise merging groups has no meaning. also this may be the reason you cannot export many objects to same file.
you can still use it this way, but without merging groups. you need to move and animate body, top and shirts together by yourself.