No Secretary of State has deleted such a large volume of e-mails in the history of the United States.
This is misleading, as emails are a relatively new innovation. Prior to 1990, it is extremely likely that no Secretary of State had sent an email. So "history of the United States" in this case is roughly twenty-five years.
Which exact e-mails is she reluctant to release?
We don't know. If we knew, then we wouldn't care nearly as much that they were deleted.
Is there anything in these e-mails that could disqualify her from being President? I wasn't sure if there could be something that may tie her e-mails to a foreign entity or something applicable to cause a conflict of interest for a POTUS (Is there such a thing?)
I would answer this similarly to how I answered the Trump taxes questions. Albeit it is slightly more possible for a disqualifying event to appear in her emails. Note the four qualification criteria:
Natural born citizen. She could have stated in an email that she was not a natural born citizen. I don't see how this would appear in a tax form. More likely to be revealed in Clinton's emails than Trump's tax forms. Even though Trump is more likely not to meet this criterion (his natural born citizen status is more questionable than Clinton's).
At least thirty-five years of age. Someone could reveal an incorrect age in an email or tax form, but it's extremely unlikely in Clinton's case (or Trump's). Their ages are pretty well documented to be well over thirty-five.
Resident for fourteen years. This could be revealed in an email, but it is extremely unlikely to be a problem for Hillary. She was a resident while Senator and Secretary of State. That leaves only the four years since leaving the State department. But how could an email from 2012 or earlier (the deleted emails were from that period) reveal her residency status in 2013 to 2016? Much less likely for Clinton's emails than Trump's taxes.
Not previously president for six or more years. It is possible that her emails reveal that Bill Clinton was just a figurehead and she was the real president from 1993 to 2001. In which case there is an argument (legally untested) that she is not eligible to be president. This is both more likely to be true in her case than Trump's and more likely to be revealed in emails than in tax forms.
The main concern here is not disqualifying information, but information that is politically negative.
Is there any legitimacy to any of the three concerns Donald Trump expressed as to why she'd not want to share her deleted e-mail information with the American people?
You only listed two concerns.
I read and heard Donald Trump say the reasons why she deleted the e-mails are (1) there could be something she wants to hide, (2) there could be more classified e-mails than she claimed...
1 seems rather obvious. If there isn't anything to hide, why has she gone to such lengths to hide them? This is an entirely legitimate criticism that Democrats, including Hillary Clinton, have used on other subjects. Turnabout is fair play.
In regards to 2, the FBI found some of the deleted emails and found additional classified material in them. That makes it legitimate to speculate that there are additional emails with classified material among the still missing emails.
In general, not releasing something makes it legitimate for people to speculate as to why. This is good and healthy, as it tends to push candidates towards revealing things rather than away. More information is often better. Consider that there was information in Nixon's tax forms in 1972 that helped in the proceedings to get him out of office.
Example answer
As written, I'd find that unrelated to the question. However, I could see it appearing as part of a longer answer if proof that Clinton lies was needed.
