Long Answer: The majority of the models use existing models and/or textures from StarCraft II as a basis. That means that the rights belong to Blizzard, at least partially.
Blizzard tolerates the usage of their material if it's being used for custom maps or mods for their games. Since it all stays within their games, the copyright isn't an issue. Also, as long as it's for free it's also fine.
However, as soon as you use the models outside of a Blizzard game (for a Unity or Unreal 4 game for instance), the above is no longer the case. You would take copyrighted material without Blizzard's consent. And no matter if it's paid or free, they probably won't allow the usage of their models for anything else.
Now, assuming that there are models that are 100% original, you would need to contact the creator and ask for permission, in addition to asking for the model in another format (I doubt that any other engine supports .m3 files). But again, I doubt that many models here on SC2mapster would qualify for fair use in a non-Blizzard game to begin with.