Do these sound natural?

yes you need the "about"

You need the "about." What meaning do you want to convey? Bullshitting means lying, which is different from shit-talking or trash-talking.

Yes

I would say Why are you talking shit? Why're you talking shit/trash talking?

I would say, "I don't know what bullshit you are talking about."

No, I’m not quite sure what you’re trying to say here

Wouldn't say any of those. "[What] the F [are] you talking about," "[What] the F [do] you mean," and "[What] the F [are] you saying" are common in the US. Some or all words in brackets may be dropped in AAVE, and sometimes by people who don't use AAVE but have some exposure to it. MLE could have similar properties, but I don't know much about it. Another powerful rejoinder is just "...what?" Often said with a hard, breathy T for emphasis. You aren't even bothering with the other words. You don't have to. It implies that the other person inherently owes you an explanation. In most North American accents, terminal T ends with a glottal stop or other silent truncation most of the time. If someone articulates it the _proper_ way, with a sibilant exhalation, it's for emphasis, and usually nothing good. Could be anything from your mother scolding you, to a street conflict where someone's liable to get hurt.

"What are you talking?" doesn't work on its own, you need "about". So "what are you trash-talking?" also needs "about".

1 and 3 are incomplete. 2 is fine. 1 and 3 would be completed if you added “about” to the end.