Hello Yung Yang,

If I correct understand you want 1 domain and 3 sub-domains for example like on flynax demo (auto.your_site.com, realty.your_site.com and general.your_site.com). Will be 1 database for all. Yes it's possible but you should contact with sales department about total price. And needs big customization for merge 3 sites in 1 database.