I know that my TextIndexNG3 is still widely used in Zope and Plone installations but it is now time to say goodbye and end the support for it. TextIndexNG3 was written with and for Python 2.7 and it will die when Python 2 support dies by the end of 2019. TextIndexNG3 makes heavy use of the Python C extension API and it would be a lot of work for porting the C code of Python 3...this is not going to happen.
TextIndexNG3 started as successor of TextIndexDS9 (my fulltext index implementations that I started in parallel to ZCTextIndex in 2001/2002 while I was working for Zope Corporation). TextIndexNG3 served us for almost 15 years from Plone 2.0 to Plone 5.2. Perhaps one of the longest running Plone add-ons. Functionality-wise TextIndexNG3 was always the better choice over ZCTextIndex but every software has its time.
TextIndexNG3 will continue with Python 2.7 based Plone sites for the time being however no further support on Plone 5.2+ under Python 3.
I strong recommend switching to Solr or Elasticsearch based implementation when you need a decent fulltext indexing solution. The standard fulltext search of Plone still works but it is far away from being decent and useful.
As said: if you need a decent search solution with quality results à la Google & Co, switch to Solr or ES.