Multiple FizzBuzz with input from file
Your code is well structured and quite easy to read and understand: this is a very good point about it.

However, there are some things that may be improved, but very few of them seem to be really critical.

main(arg)

Variables int x, int y and int n are treated separately, but indeed they are semantically related, because they describe a "FizzBuzz" to produce. A dedicated class may be created, for example:

public class FizzBuzz {

  private final int x;
  private final int y;
  private final int n;

  public FizzBuzz(int x, int y, int n) {
    this.x = x;
    this.y = y;
    this.n = n;
  }
  // getters, if needed
}


Moreover, your fizzBuzz method may then go directly into this class, named something like processInput(). The loop in main() would look like this:

for (String line : lines) {
  int[] arguments = getFizzBuzzArguments(line);
  FizzBuzz fb = new FizzBuzz(arguments[0], arguments[1], arguments[2]);
  sb.append(fb.processInput()).append('\n');
}


FizzBuzz may also have a constructor taking directly the array of input data as argument.

fizzBuzz(args)

Three points concerning the modulo divisions and the booleans isX and isY.

First, the divisors (x or y) must be validated to be different from 0, if you want to avoid ArithmeticException .

Second, they should be extracted into a dedicated method, which is easier to read:

private static boolean isMultipleOf(int dividend, int divisor) {
  return dividend % divisor == 0;
}


Third, the booleans are named poorly. They'd better sound as isFizz and isBuzz: these are their meanings.

Finally, the conditionals can be simplified:

if (isFizz || isBuzz) {
  if (isFizz) {
    sb.append("F");
  }
  if (isBuzz) {
    sb.append("B");
  }
} else {
  sb.append(i);
}


getFileLines()

Since you underline that the code in Java 7, then its facilities should be used.


Replace File with java.nio.file.Path.
Use a try-with-resources block in order to close automatically the streams opened with BufferedReader and FileReader.