As stated in my 2nd previous post, D3 limits your gear at Gear 3
if you are using a 4 speed auto gearbox. For example, you are
crusing in the middle lane on an expressway in Gear 4, then you
find this person in front driving too slowly and decide to
overtake. You could either:
1) Just leave your Gear in D position and just step on the
accelerator harder, the autobox will sense that you want more power
and automatically shifts down into Gear 3.
2) If you are not confident that you will overtake fast enough, you
could put it into D3, this is when the gearbox will 'force' the
gearbox to change into Gear 3. The electronics onboard will tune
the accelerator pressure VERY slightly so that you will have faster
response time. The difference in response is not very noticable,
espically in non performance car like your Cefiro.
In conclusion, D3 is used to force the gearbox to change into Gear
3 if you are using a 4 speed auto gearbox. It also improves
accelerator response very slightly. So, D3 is usually used for
overtaking in difficult circumstances. It is sort of changing a
manual gear box from gear 4 to gear 3. Works the same way. But if
you are really not confident, I suggest just don't medle with the
gearbox, just step on the accelerator harder if you want to
overtake. But, there is no harm trying! :)