All the examples of findElement(By.xpath) I\'ve seen search the whole page, e.g.
WebElement td = driver.findElement(By.xpath(\"//td[3]\"));
WebElement td = tr.findElement(By.xpath("/td[3]"));
If you only want to to find the child elements of the tr use a relative path not an absolute.
This should work:
int index = 3; List tds = tr.findElements(By.xpath(".//td")); System.out.println(tds[index].getText());