1.At first add 3 parameters on report (go to report >>add parameters give name as 'sum','discount' and 'total'.
2.On report right click on text box and go to expression >> click on parameters (in category)
>> double click on parameters which you want to display in your textbox .
it gives expression as
'=Parameters!sum.Value'
3. apply code below before loading report
ReportParameter[] allPar = new ReportParameter[3];
ReportParameter parSum = new ReportParameter("sum", "195");
ReportParameter parDiscount = new ReportParameter("discount", "19");
ReportParameter parTotal = new ReportParameter("total", "176");
allPar[0] = parSum;
allPar[1] = parDiscount;
allPar[2] = parTotal;
reportViewer1.LocalReport.SetParameters(allPar);
this.reportViewer1.RefreshReport();