If you've got a big lump sum to deposit for 3 years then Nationwide are offering their 3 year Fixed Rate ISA (annual interest) which is giving an interest rate of 4.1% for amounts of £50,000+ and Northern Rock are offering 4% for amounts over £500.

here's a list of the interest rates:
£1 - £9,999 3.85
£10,000 - £24,999 3.95
if you have less than £25,000 you will get a better interest rate with: Northern Rock / Savings / 3 Year Fixed Rate E-ISA which has an interest rate of 4%
£25,000 - £49,999 4.05
£50,000+ 4.10

for more information about the 3 year Fixed Rate ISA (annual interest) - savings interest rates and account summary go here:
3 year Fixed Rate ISA (annual interest) - savings interest rates and account summary from Nationwide Building Society

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more about the Northern Rock - 3 Year Fixed Rate E-ISA (Issue 6)
Northern Rock / Savings / 3 Year Fixed Rate E-ISA

Our 3 Year Fixed Rate E-ISA offers tax-free savings with a fixed interest rate throughout the term of the account online. You can only subscribe to one Cash ISA per tax year, up to the current subscription limit of £5,100 per tax year.
interest rate 4.0%