Does the Bootstrap 3 Theme for Drupal meet WCAG 2.0 and ATAG 2.0 standards?
As you can see in this bugreport: https://github.com/twbs/bootstrap/pull/11757 Bootstrap aims to be WCAG compatible. But it does not claim it already is. Also, there are a lot of WCAG guidelines that are not theme-only. The very first one:


  Guideline 1.1 Provide text alternatives for any non-text content so that it can be changed into other forms people need, such as large print, braille, speech, symbols or simpler language.


Obviously, theme cannot force you to comply with it.

Audit

WCAG is now, effectively, a part of law in European Union. This means that no one will claim to be WCAG compliant lightly, and specialized law / audit companies charge big bucks to give you a "WCAG compatible" document with their signature on it. From my experience, we talk about 4, 5 digits sums, if you need it for government cooperation purposes (like, you want to sell website to a city). Possibly more. Open Source projects hardly have funding for that, that's why you "did not find any information on the Bootstrap 3 page", and you will not in predictable future.

Pay from your own pocket, or use at your own risk.