can i use 4 3v coin battery for a 12v grain of wheat led bulbs
If it is an LED, which is doubtful, connecting them to the battery is not a good idea.
A light bulb will have its own filament resistance, so then it would be fine. 

LEDs should be driven by a constant current source. 

At a minimum you must use a resistor to limit the current.
With nothing to limit the current the battery will be drained very quickly if the LED does not fail first. 
The ohm value of the resistor should be selected carefully for efficiency.
Using a resistor you must balance efficiency and brightness. 

The forward voltage of an LED is a characteristic of the LED.
The forward voltage range for LEDs is typically 2.0-3.9 volts.
The forward voltage will vary depending on the current and the LED's junction temperature. 

There are no 12V LEDs.  A White 12V LED is actually an LED array and will have 3-4 LED chips.  Red maybe 4-5.  Depends on the  LED's forward voltage.

To select the resistor you must measure the forward voltage of the array or string of LEDs.



The input voltage to an LED circuit must be more than the forward voltage.
In the circuit below the excess voltage is dropped across the resistor.
The closer the input voltage is to the LED forward voltage the less energy that will be converted to heat by the resistor rather than light. 
LEDs are greedy, they will take all the current allowed.
So the resistor value must also limit the current.

The correct way would be to use a linear buck step down regulator.

If the input voltage is not greater than the LED forward voltage then a boost step up regulator. 
A linear regulator is very simple and very inexpensive (about $2.00). 
Below is a $1 switching regulator but if you use only analog dimming (no PWM) you do not need the inductor.  
Vin is your battery
Iadj is the current adjust which uses a single resistor
Cin is not needed when powered by battery
VCC is a 5V output.
The voltage and current of the schottky diode is dependent on the circuit in your case 20V 100mA would do.