Update Table1
Cross Join (
Select Min( Case When Z1.Num = 1 Then Z1.postcode End ) As PostCode1
, Min( Case When Z1.Num = 2 Then Z1.postcode End ) As PostCode2
, Min( Case When Z1.Num = 3 Then Z1.postcode End ) As PostCode3
From (
Select postcode
, @num := @num + 1 As Num
From postcodeTable
Where postcode = 'KY6 IDA'
Order By <equation to calculate distance> ASC
Limit 3
) As Z1
) As Z
Set nearestPostCode1 = Z.PostCode1
, nearestPostCode2 = Z.PostCode2
, nearestPostCode3 = Z.PostCode3
Where Table1.postcode = 'KY6 IDA'