string strSQL = "select sum(t1.jine)+sum(t2.jine)+sum(t3.feiyong) as total from 
biao1 t1,biao2 t2,biao3 t3
where t1.liushuihao =t2.liushuihao and t1.liushuihao = t3.liushuihao and t2.liushuihao =t3.liushuihao";
int ret = 0;

using(SqlConnection con = new SqlConnection("Data Source=.;uid=sa;pwd=sa;Database=xxx"))
{
   con.Open();
   using(SqlCommand cmd = new SqlCommand(strSQL,con))
   {
       ret = (int)cmd.ExecuteScalar();
   }
}
string s = "总金额为:"+ret.ToString();