flooring the gas pedal for the instant you need the transmission to downshift is the right procedure rather than putting the gear in D3, if you check your throttle you will see that there are 2 cables are connected, the first cable is for the gas pedal and the other cable is for the transmission throttle cable... the transmission throttle cable allows the transmission to estimate how much the driver is stepping on the gas pedal... when you suddenly floor the gas pedal, the transmission will interpret this that you are needing extra power and will automatically downshift the A/T to the next gear... this will work at any gear of the transmission

flooring the gas pedal just an instant you need the power, when you feel the A/T already downshifted, release the pedal slightly as you control the vehicles speed..

also you may use the OD switch as an alternative, assuming your gear is already on the OVER Drive 4th or 5th gear, switching the OD OFF will downshift the transmission to 3rd gear.. but the OD switch will only work if your gear is in the highest gear possible..

use only other gears like D3 S L when going uphill or towing heavy load but on flat surface, hiways, only use D,