If #3 shows up like this in your text, then it's correct, yes.
3: Several students -- twenty in all -- auditioned for the lead.
There's a big difference between how hyphens (-) are used and how dashes (--) are used. https://www.ef.edu/english-resources/english-grammar/hyphens-and-dashes/
#1 is definitely incorrect, as you have chosen.
#2 could be correct if there were a space between "children" and the opening parenthesis. However, I believe dashes would be better used there than parentheses. I agree with you that this is incorrect, but be sure to double-check your text and this: http://www.thepunctuationguide.com/em-dash.html