A list of existing and former windmills whose sites fall within Greater London, England. When built, these mills were within the counties of Kent, Surrey, Middlesex, Hertfordshire or Essex. Of those windmills known to have existed, nine remain and are preserved; as are the tide mills at Three Mills, West Ham.
There are 3 mills at Three Mills.
I think that the syntax was confusing, as the number of mills and the name of the mill were similar. However the Context gave no clue as to how many mills are actually at Three Mills.