The total rent you will pay over the term of the lease will be £45,000 (being £15,000 x 3 years).
Then you simply spread this across each accounting period. Assuming your first accounting period is a full year, you will have tax relief on £9,000 each year (being £45,000 ÷ 5 years).
So even though you won’t have physically paid any rent in the first two years, you will get tax relief on £18,000.
As these figures are fairly significant, you would be advised to discuss this further with your local TaxAssist Accountants though, to ensure that the accounts and tax treatment are correct.