How are Selenium screenshots handled with multiple instances on Grid?

It will work absolutely fine.

The screenshot is actually the image of that specific driver instance and not a genetic desktop image. You will not see multiple browsers in each screenshot

First and foremost, Selenium/WebDriver/Selenium Grid would not handle multi-threading for you, its your underlying test framework (TestNG/JUnit/Cucumber etc.) would handle it. WebDriver is not thread-safe, If you are running tests in parallel, you would need to make sure your code is thread-safe.

Going back to your question, the code you wrote would overwrite on the same screenshot file. You would need to copy the file somewhere else with a different name. I would suggest you to prefix the screenshot file with a time stamp with millisecond precision and then copy the screenshot file. This way you would have three unique different screenshots for three different browser instances. This worked for me in the past.

File scrFile = ((TakesScreenshot)driver).getScreenshotAs(OutputType.FILE);String file_name = "screenshot_"+ Add system_time with millisecond precision FileUtils.copyFile(scrFile, new File(file_name));

Here is a snippet from my Utiility code which works perfectly fine

  String path = null;try {        File source = ((TakesScreenshot)                driver).getScreenshotAs(OutputType.FILE);        Calendar currentDate = Calendar.getInstance();        SimpleDateFormat formatter = new SimpleDateFormat(                "yyyy/MMM/dd HH:mm:ss");        String dateN = formatter.format(currentDate.getTime()).replace("/","_");        String dateNow = dateN.replace(":","_");        String snapShotDirectory = Files.screenShotDirectory  + dateNow;        File f = new File(snapShotDirectory);        if(f.mkdir()){        path = f.getAbsolutePath() + "/" + source.getName();        FileUtils.copyFile(source, new File(path));         }    }    catch(IOException e) {        path = "Failed to capture screenshot: " + e.getMessage();    }

You can try using it.