They are the exact same physical remote. When you buy a second one for the Inspire 3 it is literally the same box with an "Inspire 3" sticker added.
But they have different software/firmware and are not compatible.
My dealer has tried to load the I3 software onto a m300 RC Plus but it didn't work.
The answer at the DJI forum was "no".
I think they want you to have to buy additional $1500 remotes....