Theoretically you should be able to do this but I don't know of any vendors that make imux's that can do layer 1 bundling of NxDS3 just like NxDS1. There may be some out there but I have not heard of them.
The IMA modules let you inversely multiplex ATM cell streams over more than one physical circuit, but those are T1 and E1 only as far as I know.
