Ok, thank you!!!!! :)

It looks overcrowded. They will love you more if you get them a bigger pot or individual pots for each plant. :-)

Better yet, grow sempervivum outdoors...it is a hardy succulent that appreciates winter. It will survive indoors, but usually doesn't look so great. It needs good light too - in your set up, it is shaded by other plants.

I agree with Garret except that I can't tell the the Echeveria is 'black prince,' only that it's Echeveria, and all 3 desperately need sunlight. If you don't have a south or west window, or somewhere outside for them, a mirror or a grow light might be sufficient.

What could it be if it's not black prince? And how often should I be watering it?

water needs vary hugely depending on a host of variables including pot type, pot size, growing medium, air temperature, relative humidity, growth cycle, sun exposure, season, etc. Most Echeverias need to be watered only when the soil is almost none dry if the plant is dormant (summer and winter for most species), and only when the soil is barely moist if it is actively growing (usually spring and fall if temperatures are right). To further complicate things,,not all Echeverias tolerate the same treatment. I have several community pots, and I'm forever rearranging them because one or two species in the pit would prefer to be drier or wetter than the rest.

I don't know that Garret isn't correct about your Echeveria cultivar. He probably is. Black Prince does have points on the ends of the leaves, and despite the lack of sunlight, yours seems to have a darkish hue that might turn almost black with enough sunshine. If yours were growing in enough sun, it would have a more recognizable appearance- and would be easier to ID.