Selection.FormatConditions.Add Type:=xlExpression, Formula1:=
 _"=SUMPRODUCT(($B$7:$B$100=$B7)*($C$7:$C$100=$C7)*($D$7:$D$100=$D7))>1"  
Selection.FormatConditions(Selection.FormatConditions.Count).SetFirstPriority     
With Selection.FormatConditions(1).Interior
.PatternColorIndex = xlAutomatic         
.Color = 10066431         
.TintAndShade = 0     
End With     
Selection.FormatConditions(1).StopIfTrue = False 
End Sub

 using Excel = Microsoft.Office.Interop.Excel;
 ...
 object mis = Type.Missing;

 Excel.FormatCondition cond =
    (Excel.FormatCondition)range.FormatConditions.Add(Excel.XlFormatConditionType.xlCellValue,
    Excel.XlFormatConditionOperator.xlEqual, "=SUMPRODUCT(($B$7:$B$100=$B7)*($C$7:$C$100=$C7)*($D$7:$D$100=$D7))>1",
    mis, mis, mis, mis, mis);
    cond.Interior.PatternColorIndex = Excel.Constants.xlAutomatic;
    cond.Interior.TintAndShade = 0;
    cond.Interior.Color = ColorTranslator.ToWin32(Color.White);
    cond.StopIfTrue = false;