Hi I had the same issue as described here
You must contact steinberg, or if you done so, then just wait to get back to you as you can not remove the 3rd activation by yourself, it’s something that should be done by them.
I was on the phone with them as they removed the 3rd license and at the same time I was confirming this on activation manager etc
FWIW my problem was caused by a recent hardware change, as they told me, so the product was faulty activated twice on same machine which appeared like different machines.