A lot of knime nodes have an unexpected behaviour: even if you have setted a fow variable, node check is done using main settings.
So, even if you setted correctly flow variable to cycle over files, you have to compile main form with a path to an existing dummy file (this file will be not read during process because you will set flow variable)
Pay attention! You have to set a Loop End Node after TripleReader Node. If not there will be no loop.