It's definitely No. 3, based on the fact that the other end of it is connected into the fitting that hose No. 1 connects to at the top. The mystery hose is actually pretty small - maybe 1" in diameter.

I think I'm being thrown off by the fact that it's dangling down near the bottom of the radiator in my X5. I think it actually belongs somewhere higher up - it probably fell during the course of the project. I need to remove the covers to get a better look, but it's too late tonight, so I'll remove the covers when I get home from work tomorrow. It can't be that complicated - I'm sure it will stare me right in the face once I get access to it.

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2003 X5 4.4i
1992 Porsche 968
2007 328i
2014 Subaru Forester XT