have been trying to install SAGA provider in QGIS 3 without success.
Unable to install saga in QGIS
I see two solutions for this here: https://gis.stackexchange.com/questions/87065/enabling-saga-in-qgis
(this info is 3 months old and involves ubuntu)
title: "Installing the "SAGA Next Gen" plugin in QGIS"
hope any of this gets you in the right direction. I have no experience with these apps