I will try to help I do not know what Year or Model Honda You have so I will try to explain as best I can.On all Honda's there is either a 4 speed or 5 Speed Automatic which means you have either 4 or 5 forward gears on your car. The more gears means the less RPMs you have at sustained higher speeds which translates into lower gas consumption.Most Model from 2006 on had 5 forward gears.D3 means that you have asked your car to ignore any gear beyond 3rd and increase the duration of gear for #s 1-3 giving you more torque (pulling power) and a higher RPM between gears. This is particularly helpful when you are either climbing or descending very steep hills as it eliminates the lag between gears when you have gravity fighting you on the way up and it helps to have the engine assist you in breaking on the way down.For most everyday driving you should be in D as you will get the most efficiency from your Automatic and save the most gas as you will have the proper gear for the conditions you are currently in.Hope this helps.You can contact me directly if you need more answers.