read specific column from header and get match with other file in groovy

i am new to groovy coding, kindly help .

Consider i have a excel file with below data:

first row blank, second row 1 header, third row correct header and consecutive rows has data

how to fetch data from 4th row 3rd column onwards? ( eg. t1,t2,t3 etc only)

sample code writtn so far..

Workbook workbook = WorkbookFactory.
create(new FileInputStream("/home/Desktop/testref.xlsx"));
loggerApi.info("Workbook has " + workbook.getNumberOfSheets() + " Sheets : ");
def getSheetNames() {
    int nrSheets = workbook.getNumberOfSheets();
    String[] names = new String[nrSheets];
    for ( int i = 0; i < nrSheets; i++ ) {
    names[i] = workbook.getSheetName( i );
    }
    loggerApi.info("names"+names);
 return names;
}

if (getSheetNames() == 'Reference Catalog'){

    loggerApi.info("Sheet one chosen");
}

Sheet sheet = workbook.getSheetAt(0);
loggerApi.info("Sheet"+sheet);
ExcelWSheet = workbook.getSheet('Reference Catalog');
loggerApi.info("ExcelWSheet"+ExcelWSheet);

loggerApi.info("Iterating over Rows and Columns using Iterator");
Iterator<Row> rowIterator = sheet.iterator();
while (rowIterator.hasNext()) {
    Row row = rowIterator.next();
    
    int len = row.getLastCellNum();
    for ( int i = 0; len > i ; i++) {
        
        loggerApi.info(row.getCell(i).toString());
        
        if(len-1 == i)
        {
            // print nothing
        }
        else
        {
            loggerApi.info(",");
        }
        
    }
    System.out.println();

}