Multiple selection problem in listbox

i have two feild building and unit.i want to populate unit value in one list box1.and finaly as per selection  unit value from listbox1 that unit and building value populate into two other list box..(insert building in listbox2 and insert unit in listbox3)

my code:
populate building and unit in listbox1

dtVacant =

OraDaVacant =

Library.objOracleConnection.Open();

OraDaVacant.Fill(dtVacant);

new DataTable();string strSelectVacant ="SELECT TEMC_BUILDING, TEMC_UNIT FROM TTCM_ESTATE_MASTER where TEMC_VAC_IND ='Y' order by 1";new OracleDataAdapter(strSelectVacant,Library.objOracleConnection);

lstVancantList.DataTextField = "TEMC_UNIT".Trim();

lstVancantList.DataValueField ="TEMC_BUILDING".Trim();

lstVancantList.DataBind();

lstVancantList.Items.Insert(0,"--Select Vacant List--");