They hold estates in both Hadrian and Valstiir, the house in Hadrian is small and there to control any interests the Noble Houses may need to supply the Enclave.
Hadrian once a small fishing village has grown rapidly of late after the arrival of the Valstiir Enclave, some say to fast and is affecting the local area too much.
The Archmage Valstiir has entrusted the three Noble Houses to serve/supply the Citadel and in return given land/power to use towards there own goals. Lady Valstiir cannot do everything, so relies on the houses to do their jobs but is ever vigilant.
There is no reason for another Noble House could not find a way into the Archmages fold, but you would be careful not to tread on too many toes. It might be a niche area that the others houses are not involved in heavily.
A quick/basic rundown of the current Nobles areas of influence is...
House Dukarrus - Militaristic
House Reschotti - Trade
House Skettus - Investment/Artifacts
Hope that helps