How to Convert Multiple Bodies into One One Part File - Wont/Can't Combine Bodies

I am trying to create a set of molds for my mouthwash bottle but I can't as I have 3 bodies, I try to combine them but it results in zero boundries and im stuck.

I have seen some peoples work with just an icon in the feature manager saying 'imported' with no processes included, that woule be ideal I think.

Will

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Attached are two files. One with just the 3 bodies and the other with the steps I took*

SolidworksParts And Features