Can we use battery in reverse to step down a dc voltage
No

A "perfect" battery might work for this, but any real life battery is going to have big problems.  It will work OK to begin with but the current is flowing through the battery in the wrong direction, i.e. backwards compared to when the battery is being used normally. This is the same as if the battery is being charged, so the battery will quickly become overcharged.  The consequences vary depending on the amount of current and the battery chemistry but may include:


Battery voltages much more than 3V
Producing dangerous/explosive gas
Fire, thermal runaway or explosion
Other damage to the battery


To drop 12V to 9V for a variable load, use a regulator.  To get the most stable voltage, use a linear regulator. If efficiency is more important, look for a switching regulator. Regulators are very cheap - my usual supplier has a bag of 10 suitable 100mA regulators for sixty pence (less than a dollar US).