I would guess that the one in the middle is the parent, because it's most common to place a plant in the center of a container when potting it up. Just a guess, though.
The two rather unhealthy looking offsets may
be suffering because the plant is root bound. I'm not positive of that, but there are a lot of plants in that one small pot.
Commercial potting mix is fine. It's what I always use for cacti and succulents.
Yes, water the offsets after you pot them up, but let the soil dry before watering again.
I never put gravel or anything in the bottom of my plant containers. I use a piece of non-metal screening ... the kind they use for screen doors and windows ... in the bottom, so the soil doesn't get washed out of the pot and to keep insects from crawling in. Rocks actually don't do anything much to improve drainage, and in some cases can be detrimental to the plant. Their only real value would be to increase the weight of containers if you live in a very windy area.
Just for fun (
), I thought you might like to see a picture of the flowers your Sempervivum
will produce when they're about 4 years old. Unfortunately, they only flower once, then die, but you'll have new offsets every year.
"Isn't it enough to see that a garden is beautiful without having to believe that there are fairies at the bottom of it too?" - Douglas Adams