Since there is only one line that can constitute with the requirement, we can check it directly without any loops:

private boolean isWin() {
  int playerTotal = lastPlayer * 3;
  for (int i = 0; i < SIZE; i++) {
    if (board[0][i] + board[1][i] + board[2][i] == playerTotal) {
      return true;
    } else if (board[i][0] + board[i][1] + board[i][2] == playerTotal) {
      return true;
    } 
  } 
  if (board[0][0] + board[1][1] + board[2][2] == playerTotal) { 
    return true; 
  }   
  return false; 
}