Er.. was unsavable.
If you look at the pictures the yellowing at the root wasn't enough to cut off and save..
The two others survived. Which I replanted seperate. And they are fine. :-)
I attempted to cut out the black spot I first noticed after some research with stem rot....
But a day later I came home to see this (below)
I knew it might not be able to be saved even though I repotted seperate and in a faster draining 50/50 cactus soil and sterile sand mix for faster draining (so maybe the root could dry out?)
but when I took out of the soil to check how bad the yellowing of the stem was it wasnt enough to keep it up..
I ended up having to quarantine this cactus for that 24hour test period before throwing it out when I checked the root after cutting the black spot. I didnt want it to infect the others..