I’m testing GP3.0 on a staging site.
The only bug I have found so far is with avatar images called in my author box being too small ie not honouring the dimensions set. The authorbox is created using elements.
When activating 3.0 there is no issue found with this, it only appears after switching to flexbox (which has not caused any other layout issues once browser caches are cleared).
I have included a link for you to visualise the error
the Flex version uses box-sizing: border-box – this effectively keeps padding inside the width of an element. This needs to be considered when adding a width and padding property in CSS. Your avatar CSS is this