Here is the Python code I use with QGIS 3.0 to export to an image from a layout you have previously set up and saved in your project.

        layout = QgsProject.instance().layoutManager().layoutByName("YourLayoutName")
        export = QgsLayoutExporter(layout)
        export.exportToImage("YourPath/YourFilename"+".jpg", QgsLayoutExporter.ImageExportSettings())

For a PDF export you would use:

        layout = QgsProject.instance().layoutManager().layoutByName("YourLayoutName")
        export = QgsLayoutExporter(layout)
        export.exportToPdf("YourPath/YourFilename"+".pdf", QgsLayoutExporter.PdfExportSettings())

The API is a bit confusing, and some things are broken out of the box in QGIS 3.0 but it’s gotten better with subsequent revisions. QGISLayoutExporter API Documentation. I suggest updating your version of QGIS to the latest stable version, which as of now is QGIS 3.4.3 but may be newer depending on when you find this webpage.