In 2.x versions of QGIS I was able to add LWGEOM to the QGIS Processing Toolbox and run the Make Valid command. Using OSGEO4W, I've downloaded QGIS 3.2.1 and also downloaded the latest LIBLWGEOM, but I can't see a way to add the LWGEOM tools to the Toolbox. Any ideas what I'm missing?
Because lwgeom is a Python Plugin, it must be installed from inside QGis. Further, since Qgis3 a plugins code must be valid python3 code. At least, each plugin defines which versions of QGis are supported. In the case of lwgeom, latest qgis version is 2.99 (https://plugins.qgis.org/plugins/processinglwgeomprovider/version/0.4.4/). So I recommend to contact the developer of that plugin.