England and Wales both publish an index of multiple deprivation. They are not the same measurement, and a decile in one says nothing about a decile in the other.
England's Index of Multiple Deprivation and the Welsh Index of Multiple Deprivation share a name, a decile scale running one to ten, and almost nothing else. England's combines seven domains. Wales's combines eight, assembled from 54 indicators, weighted differently.
Both rank neighbourhoods against others in the same country and only that country. A Welsh area in decile 3 is in the third-most-deprived tenth of Wales. An English area in decile 3 is in the third-most-deprived tenth of England. The two statements are built from different ingredients and measured against different populations.
Which is why area pages here branch by country. English authorities show IMD deciles and the government's sub-regional fuel poverty estimate. Welsh ones show WIMD and the Welsh Housing Conditions Survey's own figure. Welsh Government guidance is unusually direct on the point: the indices should not be used to compare deprivation between UK countries, because each country measures it differently.
Put plainly, a WIMD 3 and an IMD 3 are two different answers to two different questions that happen to have been given the same number.