Tilly, I think if you want to break out of your current low fixed rate in order to fix for longer for peace of mind, you should go for the 5 year fix. 3 years is too short, especially as you are fine now until April 2012, that is, for more than a year. So if you give that up now to go for a higher 3 year fix, you'd only really extend the fix period by less than 2 years, but will start paying more for it straight away.
In my opinion, it's not worth it. Either fix for 5 years or just stay with the current fix, but save up the difference (between repayments at 5.3% and your current repayments) into a savings account, to help you later, if interest rates do increase substantially by April 2012.
Fixing for 3 years will only have you worrying again in a year or two about what will happen when your 3 year fix finishes...
All this is IMHO, of course.