Honda Ridgeline Owner's Manual page 206

Automatic Transmission
D3 INDICATOR
When D3 is on, the transmission
selects only the first three gears.
Use D3 for engine braking when
going down a steep hill.
200
When towing a trailer on level roads
under normal driving conditions, do
not use D3. This will increase the
automatic transmission fluid
temperature and may cause
overheating.
Shifting out of the D position will
cancel D3, and cause the D3
indicator to go out. Selecting the D
position again will resume D3, and
the indicator will come on.
Turning the ignition switch to the
LOCK (0) position turns D3 off. To
use D3 when you restart the engine,
select the D position, and press the
D3 switch again.
The D3 indicator also comes on for a
few seconds when you turn the
ignition switch to the ON (II)
position.
Second (2) To shift to second,
pull the shift lever towards you, then
shift to the lower gear. This position
locks the transmission in second
gear. It does not downshift to first
gear when you come to a stop.
Use second gear:
For more power when climbing.
To increase engine braking when
going down steep hills.
For starting out on a slippery
surface or in deep snow.
To help reduce wheel spin.
When driving downhill with a
trailer.