I wouldn't count on it in the next couple of years. I would love to be surprised, but hardware as old as the MX100 had more than two WAN ports. I have repeatedly asked for more than two active WANs but have always been told that it is not on the roadmap, even though the development team know there is interest in the feature.
Realistically I understand why as I think that the load balancing between the two existing WANs needs to be more controllable, before they consider more than two active WANs.
If my answer solves your problem please click Accept as Solution so others can benefit from it.