Q:

TYPO3 tx_news - How to show related news by category?

is there an easy way to show related news by category in the news detail template?
For a single news item I want to show three related news by category. 
Ralf

A:

News extension does not offer such configuration for front-end plugin in TYPO3 back-end. Nevertheless you can quite easly achieve that by integrating plugin via TypoScript. You'll find more about it in official documentation for news extension. Anyway, this snippet might do the trick for you.  
Step 1) Integrate news extension via TypoScript
lib.newsRelatedByFirstCategory = USER
lib.newsRelatedByFirstCategory {
    userFunc = TYPO3\CMS\Extbase\Core\Bootstrap->run
    extensionName = News
    pluginName = Pi1
    vendorName = GeorgRinger

    switchableControllerActions {
        News {
            1 = list
        }
    }

    settings < plugin.tx_news.settings
    settings {
        excludeAlreadyDisplayedNews = 1
        templateLayout = 201
        relatedView = 1
        limit = 6
        useStdWrap := addToList(categories)
        categories.current = 1
        categoryConjunction = or
        overrideFlexformSettingsIfEmpty := addToList(detailPid)
        startingpoint = 59
    }
}

Step 2) Display news plugin via cObject in your news' extension template:
<f:cObject typoscriptObjectPath="lib.newsRelatedByFirstCategory">{newsItem.firstCategory.uid}</f:cObject>

It will make an instance of news plugin and display 6 news records from the first category to which belongs currently displayed news. 
Of course you need to specify startingpoint and templateLayout to match your specific TYPO3 configuration.

