I do not believe so. I can't find anything that explicitly states that it is not possible, but I do not see anything that even hints that it would be possible.
The multilink process sends the packets across the link in a round-robin type fashion. There is nothing in the protocol that would accomodate for the higher speed E3 link, so even if it would allow you to configure it as part of the bundle, I would suspect that it would either have problems due to the timing of things, or run the E3 link as slow as all the E1 links since it would be 1/6th of the round-robin with your E1 links...
