I believe it's 11531436122 'connecting line' and is the same for all 6 cylinder cars. It appears to be supplied as an assembled 3 component part.
I would expect it to be £25-£30 from a main dealer. Not common with 3 Series which will generally makes it more expensive and less easy to obtain from dealer own stock (ie may need to be ordered) and as a 3rd party part.
Header aka 'expansion' tank when you are searching.
It might be worth a look on ebay to see if there is a quality non-OEM part. Try searching - z3 (header,expansion) hose bmw. Just tried though and nothing comes up.