When I first looked at this question, I saw two possible interpretations of it. Skrooge has answered one. If the question was "can I extend it at the end of the three year period", then the answer is that you will be offered rollover rates at the end of the fixed period, which will probably include more fixed rate options. If so, you'll be able to fix again for another period, but the fixed rates you'll be offered will be the rates available in three years' time, which may be different to those available today.