You have earned a profit, you pay tax on profit.
You save for 3 years, you pay tax on 3 years profit.
You invest 3 years savings in a property and then claim for capital allowances on the properties purchase price reducing taxable profit for that year or however many years you choose to depreciate the asset by (not sure how that works in case of a deposit on property)
It adds up to be exactly the same, you just have to wait until you have all of the money in place.
Your analogy of purchasing the aston is no different to if you could purchase the property in one year or if you saved for the aston over 3 years.
Mathematically it is exactly the same.