So, my end goal is this: I want to take audio input from a 3.5mm audio cable, which is outputting sound, send it through the headphone plug into a phone, and transmit to a pair of bluetooth headphones. The phone is a samsung galaxy 7, the headphones are a pair of JBL Free, and I can't remember if the 3.5mm has 3 or 4 poles. Does this sound doable?
The biggest issue you'll encounter is that you have stereo output but only mono input (on your phones side). Source
As you can see, your phone has two speaker/audio out contacts (used to stream sound e.g. music to another device e.g. speakers) no matter what type of headphone jack you plug in, but only one contact for microphone/audio in (needed to stream music to the phone).
If you plug in your typical headphone cable you only connect to the speaker connectors and not with your microphone connector.
I want to take audio input from a 3.5mm audio cable, which is outputting sound.
If your audio source outputs only in mono you could simply dismantle a four connector cable and remap one of it's speaker contacts to the input contact.
If you want something more refined you could try something like this headphone jack to Bluetooth transmitter which is able to stream stereo to your headphones and probably in a much better quality.