The testo 317-3 is a CO only meter.
The testo 316-1 tests for natural gas (combustibles) but it has no display, just a beeper.
If you are looking for a natural gas (combustibles) leak detector with an LEL display, the most economical one is the Sensit HXG-2D.
Here is a training webinar Bill Spohn did on the product
The HXG-2D is also the least expensive one that fully complies with the BPI 1200 standard, if that is of interest to you.
Here is a training webinar on BPI 1200