I have updated TYPO3 6.1>6.2 and tx_news 2.x > 3.x, using a "classic" setup with a separate list and detail page.
Now
a) my templates can't be found
b) calling a detail page gives a 404 error, whenever RealURL is enabled (even if no postVarSets values for tx_news are set).
My personal experiences after a few hours:
Read the changelog at http://docs.typo3.org/typo3cms/extensions/news/Main/Changelog/3-0-0.html
Templating:
tx_news now features template overrides, which is great, so you don't have to clone the entire Resources/Private folder. But beware, the TS is different now, and there is an additional "s" in templateRootPaths for TS Setup, not for TS Constants (templateRootPath) though! (see http://docs.typo3.org/typo3cms/extensions/news/Main/Templating/Start/Index.html).
The templates will kick in now. I had to use both, constants AND setup paths, though.
RealURL and 404:
The templates have changed largely in tx_news. One main thing is the FAL introduction.
Update the templates from scratch. Use only the templates you modified in the old version (diff -rq will help here).
Still, a 404 may be thrown on the detail page. It's not RealURL, it's not a pid issue, it's not the caching framework. At least in my case, it was tx_news detecting an image, but being unable to handle it, so it threw that error (see errorHandling on http://docs.typo3.org/typo3cms/extensions/news/Main/Configuration/Reference/Index.html).
The reason was that in the extension config, Use FAL relations instead of news media elements [records.useFal] was set to both. Either set it to No - or set it to yes and re-enter the images in the FAL media field. I did the latter manually. Not sure if there's a migration assistant tool for that.