Thanks for the feedback!
It's not made clear, but you can swap to and from your clone freely.
So on level 3 you are supposed to use the belt on the mirror, then make a clone of yourself and lure the guards away from the portal by running in to the far corner once they start chasing you.
Once you make it to the far corner with the clone, you press R to swap back to your real self while the guards are still wailing on your clone, and you can make it in the portal easily.