I suppose こたぁ means こと, does it? Is this representing uneducated speech? I hope I haven't asked this same question before!
3. 「何してるんだよ」
I think I have had one similar to this, though, but unfortunately I can't find it. As far as I can gather, this should mean "What are you doing?", but I have difficulty understanding how something ending だよ can be a question.
2. こたぁ = ことは. I'm guessing こた works too, but I'm not totally sure about that. Also, こった = ことだ, as in いやなこった。
3. だ can be used with question words to form questions. Have you heard things like 何だ、これは? よ has an emphasizing effect on it. "What the hell are you doing?" "Just what is it you're doing?" etc.
It is slurred or sloppy speech, not necessarily 'uneducated' speech. It's 'ことは', not just 'こと'.
3. 「何してるんだよ」
I think I have had one similar to this, though, but unfortunately I can't find it. As far as I can gather, this should mean "What are you doing?", but I have difficulty understanding how something ending だよ can be a question.
It's adding a demanding emphasis.
Very like, "What are you doing!" or "What do you think you're doing?"
Very much not like, "What's that you're doing?"
The speaker probably knows or suspects what the listener is doing and is actually demanding an explanation or cessation.
It must be んなこたぁない.
His Japanese is rather stiff and translatese-like. I believe his true purpose is to post the last paragraph, i.e., advertising (as same as the one in your another thread).