Valves (and subsequently burners) have a minimum BTU output.  Due to this, there is a minimum amount of heat dispersed from each burner, and if a lower temperature can’t be achieved with all 3 burners in the low position, the next step is to turn burners off.  First, try the middle off and the two outside burners on low.  If this doesn’t achieve the desired temperature, turn the outside burners off and the middle burner on low.  If this doesn’t achieve the desired temperature, the recommendation would be to open the lid slightly to allow additional heat to escape from the chamber.

 

Please note this is a common issue among all gas grills, all brands.