Hard to tell from your picture as to the nature of your mix, but most commercially available mixes are a bit water retentive and cause root rot. I would remove that plant and check the roots, since it appears to be the only affected plant. If the roots are rotted, cut the stem back to good tissue, allow to callous for several days in a shaded area and replant.

I have used comercial "cactus & succulents" soil mix and my plants were not doing good either. After reading some posts here and asking about it, I decided to cut my soil 50% with perlite.

You could get a lot of tips about this, since everyone seems to have their own formula for that, but basically what I learnt is that that "cactus & succulents" soil mix still does not work properly and you need to add perlite (or something else) to make it less water retentive.

Also, I would take advantage of the fact that you have to change the soil/ inspect the roots and would move the two biggest ones into their own pots. I think they could use the space and althoug now this pot wouldn't look that beautiful for some time the plants would spread more and grow bigger.

Thank you guys!

Thank you guys!