Try it like this :
string myString = DrpDD.SelectedItem.Text + " " + DrpMM.SelectedItem.Text + " " + DrpYY.SelectedItem.Text ;
DateTime myDate = DateTime.ParseExact(myString, "dd MM yy",null) ;
int a = Sadapter.Insert(txtSErNo.Text, txtSfname.Text, txtSLname.Text, txtSadd.Text, txtScity.Text,txtSstate.Text,
txtSpin.Text, myDate, DrpGender.SelectedItem.ToString(), txtSMoNo.Text, txtSeid.Text, txtSUname.Text, txtSpass.Text);